Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.
The process of conversion depends on the specific situation and the intended purpose. This may be governed by regulation, contract, technical specifications or other published standards. Engineering judgment may include such factors as:
Some conversions from one system of units to another need to be exact, without increasing or decreasing the precision of the first measurement. This is sometimes called soft conversion. It does not involve changing the physical configuration of the item being measured.
By contrast, a hard conversion or an adaptive conversion may not be exactly equivalent. It changes the measurement to convenient and workable numbers and units in the new system. It sometimes involves a slightly different configuration, or size substitution, of the item.[ clarification needed ] Nominal values are sometimes allowed and used.
A conversion factor is used to change the units of a measured quantity without changing its value. The unity bracket method of unit conversion [1] consists of a fraction in which the denominator is equal to the numerator, but they are in different units. Because of the identity property of multiplication, the value of a quantity will not change as long as it is multiplied by one. [2] Also, if the numerator and denominator of a fraction are equal to each other, then the fraction is equal to one. So as long as the numerator and denominator of the fraction are equivalent, they will not affect the value of the measured quantity.
The following example demonstrates how the unity bracket method [3] is used to convert the rate 5 kilometers per second to meters per second. The symbols km, m, and s represent kilometer, meter, and second, respectively.
Thus, it is found that 5 kilometers per second is equal to 5000 meters per second.
There are many conversion tools. They are found in the function libraries of applications such as spreadsheets databases, in calculators, and in macro packages and plugins for many other applications such as the mathematical, scientific and technical applications.
There are many standalone applications that offer the thousands of the various units with conversions. For example, the free software movement offers a command line utility GNU units for Linux and Windows.
In the cases where non-SI units are used, the numerical calculation of a formula can be done by first working out the pre-factor, and then plug in the numerical values of the given/known quantities.
For example, in the study of Bose–Einstein condensate, [4] atomic mass m is usually given in daltons, instead of kilograms, and chemical potential μ is often given in Boltzmann constant times nanokelvin. The condensate's healing length is given by:
For a 23Na condensate with chemical potential of (Boltzmann constant times) 128 nK, the calculation of healing length (in microns) can be done in two steps:
Assume that this gives
which is our pre-factor.
Now, make use of the fact that . With , .
This method is especially useful for programming and/or making a worksheet, where input quantities are taking multiple different values; For example, with the pre-factor calculated above, it's very easy to see that the healing length of 174Yb with chemical potential 20.3 nK is .
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This article gives lists of conversion factors for each of a number of physical quantities, which are listed in the index. For each physical quantity, a number of different units (some only of historical interest) are shown and expressed in terms of the corresponding SI unit. Conversions between units in the metric system are defined by their prefixes (for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10−6 metre). Within each table, the units are listed alphabetically, and the SI units (base or derived) are highlighted.
Symbol | Definition |
---|---|
≡ | exactly equal |
≈ | approximately equal to |
digits | indicates that digits repeat infinitely (e.g. 8.294369 corresponds to 8.294369369369369...) |
(H) | of chiefly historical interest |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
ångström | Å | ≡ 1×10−10 m | ≡ 0.1 nm |
astronomical unit | au | ≡ 149597870700 m ≈ Distance from Earth to Sun | ≡ 149597870700 m [5] |
attometre | am | ≡ 1×10−18 m | ≡ 1×10−18 m |
barleycorn (H) | = 1⁄3 in (see note above about rounding) | ≈ 8.46×10−3 m | |
bohr, atomic unit of length | a0 | = Bohr radius of hydrogen | ≈ 5.2917721092(17)×10−11 m [6] |
cable length (imperial) | ≡ 608 ft | ≈ 185.3184 m | |
cable length (International) | ≡ 1⁄10 nmi | ≡ 185.2 m | |
cable length (US) | ≡ 720 ft | = 219.456 m | |
chain (Gunter's; Surveyor's) | ch | ≡ 66 ft (US) ≡ 4 rods [7] | ≈ 20.11684 m |
cubit (H) | ≡ Distance from fingers to elbow ≈ 18 in | ≈ 0.5 m | |
ell (H) | ell | ≡ 45 in [8] (In England usually) | = 1.143 m |
fathom | ftm | ≡ 6 ft [8] | = 1.8288 m |
femtometre | fm | ≡ 1×10−15 m | ≡ 1×10−15 m |
fermi | fm | ≡ 1×10−15 m [8] | ≡ 1×10−15 m |
finger | ≡ 7⁄8 in | = 0.022225 m | |
finger (cloth) | ≡ 4 1⁄2 in | = 0.1143 m | |
foot (Benoît) (H) | ft (Ben) | ≈ 0.304799735 m | |
foot (Cape) (H) | Legally defined as 1.033 English feet in 1859 | ≈ 0.314858 m | |
foot (Clarke's) (H) | ft (Cla) | ≈ 0.3047972654 m | |
foot (Indian) (H) | ft Ind | ≈ 0.304799514 m | |
foot, metric | mf | ≡ 300 mm | ≡ 0.3 m |
foot, metric (Mesures usuelles) (H) | ≡ 1⁄3 m | ≡ 0.3 m | |
foot (International) | ft | ≡ 0.3048 m ≡ 1⁄3 yd ≡ 12 inches | ≡ 0.3048 m |
foot (Sear's) (H) | ft (Sear) | ≈ 0.30479947 m | |
foot (US Survey) | ft (US) | ≡ 1200⁄3937 m [9] | ≈ 0.304800610 m |
french; charriere | F | ≡ 1⁄3 mm | = 0.3×10−3 m |
furlong | fur | ≡ 10 chains = 660 ft = 220 yd [8] | = 201.168 m |
hand | ≡ 4 in [8] | ≡ 0.1016 m | |
inch (International) | in | ≡ 2.54 cm ≡ 1⁄36 yd ≡ 1⁄12 ft | ≡ 0.0254 m |
league (land) | lea | ≈ 1 hour walk, Currently defined in US as 3 Statute miles, [7] but historically varied from 2 to 9 km | ≈ 4828 m |
light-day | ≡ 24 light-hours | ≡ 2.59020683712×1013 m | |
light-hour | ≡ 60 light-minutes | ≡ 1.0792528488×1012 m | |
light-minute | ≡ 60 light-seconds | ≡ 1.798754748×1010 m | |
light-second | ≡ Distance light travels in one second in vacuum | ≡ 299792458 m | |
light-year | ly | ≡ Distance light travels in vacuum in 365.25 days [10] | ≡ 9.4607304725808×1015 m |
line | ln | ≡ 1⁄12 in [11] | = 0.002116 m |
link (Gunter's; Surveyor's) | lnk | ≡ 1⁄100 ch [8] ≡ 0.66 ft (US) ≡ 7.92 in | ≈ 0.2011684 m |
link (Ramsden's; Engineer's) | lnk | ≡ 1 ft [8] | = 0.3048 m |
metre (SI base unit) (meter) | m | ≡ Distance light travels in 1⁄299792458 of a second in vacuum. [12] ≈ 1⁄10000000 of the distance from equator to pole. | (SI base unit) |
mickey | ≡ 1⁄200 in | = 1.27×10−4 m | |
micrometre (old: micron) | μ; μm | ≡ 1×10−6 m | ≡ 1×10−6 m |
mil; thou | mil | ≡ 1×10−3 in | = 2.54×10−5 m |
mil (Sweden and Norway) | mil | ≡ 10 km | = 10000 m |
mile (geographical) (H) | ≡ 6082 ft | = 1853.7936 m | |
mile (international) | mi | ≡ 80 chains ≡ 5280 ft ≡ 1760 yd | ≡ 1609.344 m |
mile (tactical or data) | ≡ 6000 ft | ≡ 1828.8 m | |
mile (telegraph) (H) | mi | ≡ 6087 ft | = 1855.3176 m |
mile (US Survey) | mi | ≡ 5280 US Survey feet ≡ (5280 × 1200⁄3937) m | ≈ 1609.347219 m |
nail (cloth) | ≡ 2 1⁄4 in [8] | = 0.05715 m | |
nanometre | nm | ≡ 1×10−9 m | ≡ 1×10−9 m |
nautical league | NL; nl | ≡ 3 nmi [8] | = 5556 m |
nautical mile (Admiralty) | NM (Adm); nmi (Adm) | = 6080 ft | = 1853.184 m |
nautical mile (international) | NM; nmi | ≡ 1852 m [13] | ≡ 1852 m |
nautical mile (US pre 1954) | ≡ 1853.248 m | ≡ 1853.248 m | |
pace | ≡ 2.5 ft [8] | = 0.762 m | |
palm | ≡ 3 in [8] | = 0.0762 m | |
parsec | pc | Distant point with a parallax shift of one arc second from a base of one astronomical unit. ≡ 648000/π AU [14] [15] | ≈ 30856775814913700 m [16] |
pica | ≡ 12 points | Dependent on point measures. | |
picometre | pm | ≡ 1×10−12 m | ≡ 1×10−12 m |
point (American, English) [17] [18] | pt | ≡ 1⁄72.272 in | ≈ 0.000351450 m |
point (Didot; European) [18] [19] | pt | ≡ 1⁄12 × 1⁄72 of pied du roi; After 1878: ≡ 5⁄133 cm | ≈ 0.00037597 m; After 1878: ≈ 0.00037593985 m |
point (PostScript) [17] | pt | ≡ 1⁄72 in | = 0.0003527 m |
point (TeX) [17] | pt | ≡ 1⁄72.27 in | = 0.0003514598 m |
quarter | ≡ 1⁄4 yd | = 0.2286 m | |
rod; pole; perch (H) | rd | ≡ 16 1⁄2 ft | = 5.0292 m |
rope (H) | rope | ≡ 20 ft [8] | = 6.096 m |
shaku (Japan) | ≡ 10/33 m | ≈ 0.303 0303 m | |
span (H) | ≡ 9 in [8] | = 0.2286 m | |
spat [20] | ≡ 1×1012 m | ||
stick (H) | ≡ 2 in | = 0.0508 m | |
toise (French, post 1667) (H) | T | ≡ 27000/13853 m | ≈ 1.949 0363 m |
twip | twp | ≡ 1⁄1440 in | = 1.7638×10−5 m |
x unit; siegbahn | xu | ≈ 1.0021×10−13 m [8] | |
yard (International) | yd | ≡ 0.9144 m [9] ≡ 3 ft ≡ 36 in | ≡ 0.9144 m |
yoctometre | ym | ≡ 1×10−24 m | ≡ 1×10−24 m |
zeptometre | zm | ≡ 1×10−21 m | ≡ 1×10−21 m |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
acre (international) | ac | ≡ 1 ch × 10 ch = 4840 sq yd | ≡ 4046.8564224 m2 |
acre (US survey) | ac | ≡ 10 sq ch = 4840 sq yd, also 43560 sq ft | ≈ 4046.873 m2 [21] |
are | a | ≡ 100 m2 | ≡ 100 m2 |
barn | b | ≡ 10−28 m2 | ≡ 10−28 m2 |
barony | ≡ 4000 ac | ≡ 1.61874256896×107 m2 | |
board | bd | ≡ 1 in × 1 ft | ≡ 7.74192×10−3 m2 |
boiler horsepower equivalent direct radiation | bhp EDR | ≡ 1 ft2 × 1 bhp / (240 BTUIT/h) | ≈ 12.958174 m2 |
circular inch | circ in | ≡ π⁄4 sq in | ≈ 5.067075×10−4 m2 |
circular mil; circular thou | circ mil | ≡ π⁄4 mil2 | ≈ 5.067075×10−10 m2 |
cord | ≡ 192 bd | ≡ 1.48644864 m2 | |
cuerda (PR Survey) | cda | ≡ 1 cda x 1 cda = 0.971222 acre | ≡ 3930.395625 m2 |
dunam | ≡ 1000 m2 | ≡ 1000 m2 | |
guntha (India) | ≡ 121 sq yd | ≈ 101.17 m2 | |
hectare | ha | ≡ 10000 m2 | ≡ 10000 m2 |
hide | ≈ 120 ac (variable) | ≈ 5×105 m2 | |
rood | ro | ≡ 1⁄4 ac | = 1011.7141056 m2 |
ping | ≡ 20⁄11 m × 20⁄11 m | ≈ 3.306 m2 | |
section | ≡ 1 mi × 1 mi | = 2.589988110336×106 m2 | |
shed | ≡ 10−52 m2 | = 10−52 m2 | |
square (roofing) | ≡ 10 ft × 10 ft | = 9.290304 m2 | |
square chain (international) | sq ch | ≡ 66 ft × 66 ft = 1⁄10 ac | ≡ 404.68564224 m2 |
square chain (US Survey) | sq ch | ≡ 66 ft (US) × 66 ft (US) = 1⁄10 US survey acre | ≈ 404.6873 m2 |
square foot | sq ft | ≡ 1 ft × 1 ft | ≡ 9.290304×10−2 m2 |
square foot (US Survey) | sq ft | ≡ 1 ft (US) × 1 ft (US) | ≈ 9.2903411613275×10−2 m2 |
square inch | sq in | ≡ 1 in × 1 in | ≡ 6.4516×10−4 m2 |
square kilometre | km2 | ≡ 1 km × 1 km | = 106 m2 |
square link (Gunter's)(International) | sq lnk | ≡ 1 lnk × 1 lnk ≡ 0.66 ft × 0.66 ft | = 4.0468564224×10−2 m2 |
square link (Gunter's)(US Survey) | sq lnk | ≡ 1 lnk × 1 lnk ≡ 0.66 ft (US) × 0.66 ft (US) | ≈ 4.046872×10−2 m2 |
square link (Ramsden's) | sq lnk | ≡ 1 lnk × 1 lnk ≡ 1 ft × 1 ft | = 0.09290304 m2 |
square metre (SI unit) | m2 | ≡ 1 m × 1 m | = 1 m2 |
square mil; square thou | sq mil | ≡ 1 mil × 1 mil | = 6.4516×10−10 m2 |
square mile | sq mi | ≡ 1 mi × 1 mi | ≡ 2.589988110336×106 m2 |
square mile (US Survey) | sq mi | ≡ 1 mi (US) × 1 mi (US) | ≈ 2.58999847×106 m2 |
square rod/pole/perch | sq rd | ≡ 1 rd × 1 rd | = 25.29285264 m2 |
square yard (International) | sq yd | ≡ 1 yd × 1 yd | ≡ 0.83612736 m2 |
stremma | ≡ 1000 m2 | = 1000 m2 | |
township | ≡ 36 sq mi (US) | ≈ 9.323994×107 m2 | |
yardland | ≈ 30 ac | ≈ 1.2×105 m2 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
acre-foot | ac ft | ≡ 1 ac x 1 ft = 43560 cu ft | = 1233.48183754752 m3 |
acre-inch | ≡ 1 ac × 1 in | = 102.79015312896 m3 | |
barrel (imperial) | bl (imp) | ≡ 36 gal (imp) | = 0.16365924 m3 |
barrel (petroleum); archaic blue-barrel | bl; bbl | ≡ 42 gal (US) | = 0.158987294928 m3 |
barrel (US dry) | bl (US) | ≡ 105 qt (US) = 105/32 bu (US lvl) | = 0.115628198985075 m3 |
barrel (US fluid) | fl bl (US) | ≡ 31 1⁄2 gal (US) | = 0.119240471196 m3 |
board-foot | bdft | ≡ 144 cu in | ≡ 2.359737216×10−3 m3 |
bucket (imperial) | bkt | ≡ 4 gal (imp) | = 0.01818436 m3 |
bushel (imperial) | bu (imp) | ≡ 8 gal (imp) | = 0.03636872 m3 |
bushel (US dry heaped) | bu (US) | ≡ 1 1⁄4 bu (US lvl) | = 0.0440488377086 m3 |
bushel (US dry level) | bu (US lvl) | ≡ 2150.42 cu in | = 0.03523907016688 m3 |
butt, pipe | ≡ 126 gal (US) (wine) | = 0.476961884784 m3 | |
coomb | ≡ 4 bu (imp) | = 0.14547488 m3 | |
cord (firewood) | ≡ 8 ft × 4 ft × 4 ft | = 3.624556363776 m3 | |
cord-foot | ≡ 16 cu ft | = 0.453069545472 m3 | |
cubic fathom | cu fm | ≡ 1 fm × 1 fm × 1 fm | = 6.116438863872 m3 |
cubic foot | ft3 | ≡ 1 ft × 1 ft × 1 ft | ≡ 0.028316846592 m3 |
cubic inch | in3 | ≡ 1 in × 1 in × 1 in | ≡ 16.387064×10−6 m3 |
cubic metre (SI unit) | m3 | ≡ 1 m × 1 m × 1 m | ≡ 1 m3 |
cubic mile | cu mi | ≡ 1 mi × 1 mi × 1 mi | ≡ 4168181825.440579584 m3 |
cubic yard | yd3 | ≡ 27 cu ft | ≡ 0.764554857984 m3 |
cup (breakfast) | ≡ 10 fl oz (imp) | = 284.130625×10−6 m3 | |
cup (Canadian) | c (CA) | ≡ 8 fl oz (imp) | = 227.3045×10−6 m3 |
cup (metric) | c | ≡ 250.0×10−6 m3 | ≡ 250.0×10−6 m3 |
cup (US customary) | c (US) | ≡ 8 US fl oz ≡ 1⁄16 gal (US) | = 236.5882365×10−6 m3 |
cup (US food nutrition labeling) | c (US) | ≡ 240 mL [22] | = 2.4×10−4 m3 |
dash (imperial) | ≡ 1⁄384 gi (imp) = 1⁄2 pinch (imp) | = 369.961751302083×10−9 m3 | |
dash (US) | ≡ 1⁄96 US fl oz = 1⁄2 US pinch | = 308.057599609375×10−9 m3 | |
dessertspoon (imperial) | ≡ 1⁄12 gi (imp) | = 11.8387760416×10−6 m3 | |
drop (imperial) | gtt | ≡ 1⁄288 fl oz (imp) | = 98.6564670138×10−9 m3 |
drop (imperial) (alt) | gtt | ≡ 1⁄1824 gi (imp) | ≈ 77.886684×10−9 m3 |
drop (medical) | ≡ 1⁄12 mL | = 83.3×10−9 m3 | |
drop (metric) | ≡ 1⁄20 mL | = 50.0×10−9 m3 | |
drop (US) | gtt | ≡ 1⁄360 US fl oz | = 82.14869322916×10−9 m3 |
drop (US) (alt) | gtt | ≡ 1⁄456 US fl oz | ≈ 64.85423149671×10−9 m3 |
drop (US) (alt) | gtt | ≡ 1⁄576 US fl oz | ≈ 51.34293326823×10−9 m3 |
fifth | ≡ 1⁄5 US gal | = 757.0823568×10−6 m3 | |
firkin | ≡ 9 gal (imp) | = 0.04091481 m3 | |
fluid drachm (imperial) | fl dr | ≡ 1⁄8 fl oz (imp) | = 3.5516328125×10−6 m3 |
fluid dram (US); US fluidram | fl dr | ≡ 1⁄8 US fl oz | = 3.6966911953125×10−6 m3 |
fluid scruple (imperial) | fl s | ≡ 1⁄24 fl oz (imp) | = 1.18387760416×10−6 m3 |
gallon (beer) | beer gal | ≡ 282 cu in | = 4.621152048×10−3 m3 |
gallon (imperial) | gal (imp) | ≡ 4.54609 L | ≡ 4.54609×10−3 m3 |
gallon (US dry) | gal (US) | ≡ 1⁄8 bu (US lvl) | = 4.40488377086×10−3 m3 |
gallon (US fluid; Wine) | gal (US) | ≡ 231 cu in | ≡ 3.785411784×10−3 m3 |
gill (imperial); Noggin | gi (imp); nog | ≡ 5 fl oz (imp) | = 142.0653125×10−6 m3 |
gill (US) | gi (US) | ≡ 4 US fl oz | = 118.29411825×10−6 m3 |
hogshead (imperial) | hhd (imp) | ≡ 2 bl (imp) | = 0.32731848 m3 |
hogshead (US) | hhd (US) | ≡ 2 fl bl (US) | = 0.238480942392 m3 |
jigger (bartending) | ≡ 1 1⁄2 US fl oz | ≈ 44.36×10−6 m3 | |
kilderkin | ≡ 18 gal (imp) | = 0.08182962 m3 | |
lambda | λ | ≡ 1 mm3 | = 1×10−9 m3 |
last | ≡ 80 bu (imp) | = 2.9094976 m3 | |
litre (liter) | L or l | ≡ 1 dm3 [23] | ≡ 0.001 m3 |
load | ≡ 50 cu ft | = 1.4158423296 m3 | |
minim (imperial) | min | ≡ 1⁄480 fl oz (imp) = 1/60 fl dr (imp) | = 59.1938802083×10−9 m3 |
minim (US) | min | ≡ 1⁄480 US fl oz = 1⁄60 US fl dr | = 61.611519921875×10−9 m3 |
ounce (fluid imperial) | fl oz (imp) | ≡ 1⁄160 gal (imp) | ≡ 28.4130625×10−6 m3 |
ounce (fluid US customary) | US fl oz | ≡ 1⁄128 gal (US) | ≡ 29.5735295625×10−6 m3 |
ounce (fluid US food nutrition labeling) | US fl oz | ≡ 30 mL [22] | ≡ 3×10−5 m3 |
peck (imperial) | pk | ≡ 2 gal (imp) | = 9.09218×10−3 m3 |
peck (US dry) | pk | ≡ 1⁄4 US lvl bu | = 8.80976754172×10−3 m3 |
perch | per | ≡ 16 1⁄2 ft × 1 1⁄2 ft × 1 ft | = 0.700841953152 m3 |
pinch (imperial) | ≡ 1⁄192 gi (imp) = 1/16 tsp (imp) | = 739.92350260416×10−9 m3 | |
pinch (US) | ≡ 1⁄48 US fl oz = 1/16 US tsp | = 616.11519921875×10−9 m3 | |
pint (imperial) | pt (imp) | ≡ 1⁄8 gal (imp) | = 568.26125×10−6 m3 |
pint (US dry) | pt (US dry) | ≡ 1⁄64 bu (US lvl) ≡ 1⁄8 gal (US dry) | = 550.6104713575×10−6 m3 |
pint (US fluid) | pt (US fl) | ≡ 1⁄8 gal (US) | = 473.176473×10−6 m3 |
pony | ≡ 3⁄4 US fl oz | = 22.180147171875×10−6 m3 | |
pottle; quartern | ≡ 1⁄2 gal (imp) = 80 fl oz (imp) | = 2.273045×10−3 m3 | |
quart (imperial) | qt (imp) | ≡ 1⁄4 gal (imp) | = 1.1365225×10−3 m3 |
quart (US dry) | qt (US) | ≡ 1⁄32 bu (US lvl) = 1⁄4 gal (US dry) | = 1.101220942715×10−3 m3 |
quart (US fluid) | qt (US) | ≡ 1⁄4 gal (US fl) | = 946.352946×10−6 m3 |
quarter; pail | ≡ 8 bu (imp) | = 0.29094976 m3 | |
register ton | ≡ 100 cu ft | = 2.8316846592 m3 | |
sack (US) | ≡ 3 bu (US lvl) | = 0.10571721050064 m3 | |
seam | ≡ 8 bu [20] | = 0.29095 m3 | |
shot (US) | usually 1.5 US fl oz [20] | ≈ 44.4×10−6 m3 | |
strike (imperial) | ≡ 2 bu (imp) | = 0.07273744 m3 | |
strike (US) | ≡ 2 bu (US lvl) | = 0.07047814033376 m3 | |
tablespoon (Australian metric) | ≡ 20.0×10−6 m3 | ||
tablespoon (Canadian) | tbsp | ≡ 1⁄2 fl oz (imp) | = 14.20653125×10−6 m3 |
tablespoon (imperial) | tbsp | ≡ 5⁄8 fl oz (imp) | = 17.7581640625×10−6 m3 |
tablespoon (metric) | ≡ 15×10−6 m3 | ||
tablespoon (US customary) | tbsp | ≡ 1⁄2 US fl oz | = 14.78676478125×10−6 m3 |
tablespoon (US food nutrition labeling) | tbsp | ≡ 15 mL [22] | = 15×10−6 m3 |
teaspoon (Canadian) | tsp | ≡ 1⁄6 fl oz (imp) | = 4.735510416×10−6 m3 |
teaspoon (imperial) | tsp | ≡ 1⁄24 gi (imp) | = 5.91938802083×10−6 m3 |
teaspoon (metric) | ≡ 5.0×10−6 m3 | ≡ 5.0×10−6 m3 | |
teaspoon (US customary) | tsp | ≡ 1⁄6 US fl oz | = 4.92892159375×10−6 m3 |
teaspoon (US food nutrition labeling) | tsp | ≡ 5 mL [22] | = 5×10−6 m3 |
timber foot | ≡ 1 cu ft | = 0.028316846592 m3 | |
ton (displacement) | ≡ 35 cu ft | = 0.99108963072 m3 | |
ton (freight) | ≡ 40 cu ft | = 1.13267386368 m3 | |
ton (water) | ≡ 28 bu (imp) | = 1.01832416 m3 | |
tun | ≡ 252 gal (wine) | = 0.953923769568 m3 | |
wey (US) | ≡ 40 bu (US lvl) | = 1.4095628066752 m3 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
NATO mil | (various) | ≡ 2π⁄6400 rad | ≈ 0.981748×10−3 rad |
Swedish streck | ≡ 2π⁄6300 rad | ≈ 0.997302×10−3 rad | |
milliradian | mrad | ≡ 1⁄1000 rad | ≈ 1×10−3 rad |
Warsaw Pact mil | ≡ 2π⁄6000 rad | ≈ 1.047167×10−3 rad | |
arcminute; MOA | ' | ≡ 1°⁄60 | ≈ 0.290888×10−3 rad |
arcsecond | " | ≡ 1°⁄3600 | ≈ 4.848137×10−6 rad |
centesimal minute of arc | ' | ≡ 1⁄100 grad | ≈ 0.157080×10−3 rad |
centesimal second of arc | " | ≡ 1⁄10000 grad | ≈ 1.570796×10−6 rad |
degree (of arc) | ° | ≡ 1⁄360 of a revolution ≡ π⁄180 rad | ≈ 17.453293×10−3 rad |
grad; gradian; gon | grad | ≡ 1⁄400 of a revolution ≡ π⁄200 rad ≡ 0.9° | ≈ 15.707963×10−3 rad |
octant | ≡ 45° | ≈ 0.785398 rad | |
quadrant | ≡ 90° | ≈ 1.570796 rad | |
radian (SI unit) | rad | The angle subtended at the center of a circle by an arc whose length is equal to the circle's radius. One full revolution encompasses 2π radians. | = 1 rad |
sextant | ≡ 60° | ≈ 1.047198 rad | |
sign | ≡ 30° | ≈ 0.523599 rad |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
spat | ≡ 4π sr [20] – The solid angle subtended by a sphere at its centre. | ≈ 12.56637 sr | |
square degree | deg2; sq.deg.; (°)2 | ≡ (π⁄180)2 sr | ≈ 0.30462×10−3 sr |
steradian (SI unit) | sr | The solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r2. A sphere subtends 4π sr. [20] | = 1 sr |
Notes:
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atomic mass unit, unified | u; AMU | Same as dalton (see below) | ≈ 1.660539040(20)×10−27 kg [7] |
atomic unit of mass, electron rest mass | me | ≈ 9.10938291(40)×10−31 kg [24] | |
bag (coffee) | ≡ 60 kg | = 60 kg | |
bag (Portland cement) | ≡ 94 lb av | = 42.63768278 kg | |
barge | ≡ 22 1⁄2 short ton | = 20411.65665 kg | |
carat | kt | ≡ 3 1⁄6 gr | = 205.1965483 mg |
carat (metric) | ct | ≡ 200 mg | = 200 mg |
clove | ≡ 8 lb av | = 3.62873896 kg | |
crith | ≡ mass of 1 L of hydrogen gas at STP | ≈ 89.9349 mg | |
dalton | Da | 1/12 the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest | ≈ 1.660538921(73)×10−27 kg [7] |
dram (apothecary; troy) | dr t | ≡ 60 gr | = 3.8879346 g |
dram (avoirdupois) | dr av | ≡ 27 11⁄32 gr | = 1.7718451953125 g |
electronvolt | eV | ≡ 1 eV (energy unit) / c 2 | = 1.78266184(45)×10−36 kg [7] |
gamma | γ | ≡ 1 μg | = 1 μg |
grain | gr | ≡ 1⁄7000 lb av | ≡ 64.79891 mg |
grave | gv. | grave was the original name of the kilogram | ≡ 1 kg |
hundredweight (long) | long cwt or cwt | ≡ 112 lb av | = 50.80234544 kg |
hundredweight (short); cental | sh cwt | ≡ 100 lb av | = 45.359237 kg |
kilogram (kilogramme) | kg | ≈ mass of the prototype near Paris ≈ mass of 1 litre of water | (SI base unit) [12] |
kip | kip | ≡ 1000 lb av | = 453.59237 kg |
mark | ≡ 8 oz t | = 248.8278144 g | |
mite | ≡ 1⁄20 gr | = 3.2399455 mg | |
mite (metric) | ≡ 1⁄20 g | = 50 mg | |
ounce (apothecary; troy) | oz t | ≡ 1⁄12 lb t | = 31.1034768 g |
ounce (avoirdupois) | oz av | ≡ 1⁄16 lb | = 28.349523125 g |
ounce (US food nutrition labelling) | oz | ≡ 28 g [22] | = 28 g |
pennyweight | dwt; pwt | ≡ 1⁄20 oz t | = 1.55517384 g |
point | ≡ 1⁄100 ct | = 2 mg | |
pound (avoirdupois) | lb av | ≡ 0.45359237 kg = 7000 grains | ≡ 0.45359237 kg |
pound (metric) | ≡ 500 g | = 500 g | |
pound (troy) | lb t | ≡ 5760 grains | = 0.3732417216 kg |
quarter (imperial) | ≡ 1⁄4 long cwt = 2 st = 28 lb av | = 12.70058636 kg | |
quarter (informal) | ≡ 1⁄4 short ton | = 226.796185 kg | |
quarter, long (informal) | ≡ 1⁄4 long ton | = 254.0117272 kg | |
quintal (metric) | q | ≡ 100 kg | = 100 kg |
scruple (apothecary) | s ap | ≡ 20 gr | = 1.2959782 g |
sheet | ≡ 1⁄700 lb av | = 647.9891 mg | |
slug; geepound; hyl | slug | ≡ 1 ɡ0 × 1 lb av × 1 s2/ft | ≈ 14.593903 kg |
stone | st | ≡ 14 lb av | = 6.35029318 kg |
ton, assay (long) | AT | ≡ 1 mg × 1 long ton ÷ 1 oz t | = 32.6 g |
ton, assay (short) | AT | ≡ 1 mg × 1 short ton ÷ 1 oz t | = 29.16 g |
ton, long | long tn or ton | ≡ 2240 lb | = 1016.0469088 kg |
ton, short | sh tn | ≡ 2000 lb | = 907.18474 kg |
tonne (mts unit) | t | ≡ 1000 kg | = 1000 kg |
wey | ≡ 252 lb = 18 st | = 114.30527724 kg (variants exist) | |
Zentner | Ztr. | Definitions vary. [20] [25] |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
gram per millilitre | g/mL | ≡ g/mL | = 1000 kg/m3 |
kilogram per cubic metre (SI unit) | kg/m3 | ≡ kg/m3 | = 1 kg/m3 |
kilogram per litre | kg/L | ≡ kg/L | = 1000 kg/m3 |
ounce (avoirdupois) per cubic foot | oz/ft3 | ≡ oz/ft3 | ≈ 1.001153961 kg/m3 |
ounce (avoirdupois) per cubic inch | oz/in3 | ≡ oz/in3 | ≈ 1.729994044×103 kg/m3 |
ounce (avoirdupois) per gallon (imperial) | oz/gal | ≡ oz/gal | ≈ 6.236023291 kg/m3 |
ounce (avoirdupois) per gallon (US fluid) | oz/gal | ≡ oz/gal | ≈ 7.489151707 kg/m3 |
pound (avoirdupois) per cubic foot | lb/ft3 | ≡ lb/ft3 | ≈ 16.01846337 kg/m3 |
pound (avoirdupois) per cubic inch | lb/in3 | ≡ lb/in3 | ≈ 2.767990471×104 kg/m3 |
pound (avoirdupois) per gallon (imperial) | lb/gal | ≡ lb/gal | ≈ 99.77637266 kg/m3 |
pound (avoirdupois) per gallon (US fluid) | lb/gal | ≡ lb/gal | ≈ 119.8264273 kg/m3 |
slug per cubic foot | slug/ft3 | ≡ slug/ft3 | ≈ 515.3788184 kg/m3 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
Atomic unit of time | au | ≡ a0/(α⋅c) | ≈ 2.418884254×10−17 s |
Callippic cycle | ≡ 441 mo (hollow) + 499 mo (full) = 76 a of 365.25 d | = 2.396736 Gs or 2.3983776 Gs [note 1] | |
Century | c | ≡ 100 years (100 a) | = 3.1556952 Gs [note 2] [note 3] |
Day | d | = 24 h = 1440 min | = 86.4 ks [note 3] |
Day (sidereal) | d | ≡ Time needed for the Earth to rotate once around its axis, determined from successive transits of a very distant astronomical object across an observer's meridian (International Celestial Reference Frame) | ≈ 86.1641 ks |
Decade | dec | ≡ 10 years (10 a) | = 315.569520 Ms [note 2] [note 3] |
Fortnight | fn | ≡ 2 wk | = 1.2096 Ms [note 3] |
Helek | ≡ 1⁄1080 h | = 3.3 s | |
Hipparchic cycle | ≡ 4 Callippic cycles - 1 d | = 9.593424 Gs | |
Hour | h | ≡ 60 min | = 3.6 ks [note 3] |
Jiffy | j | ≡ 1⁄60 s | = 16.6 ms |
Jiffy (alternative) | ja | ≡ 1⁄100 s | = 10 ms |
Ke (quarter of an hour) | ≡ 1⁄4 h = 1⁄96 d = 15 min | = 900 s | |
Ke (traditional) | ≡ 1⁄100 d = 14.4 min | = 864 s | |
Lustre; Lustrum | ≡ 5 a of 365 d [note 4] | = 157.68 Ms | |
Metonic cycle; enneadecaeteris | ≡ 110 mo (hollow) + 125 mo (full) = 6940 d ≈ 19 a | = 599.616 Ms | |
Millennium | ≡ 1000 years (1000 a) | = 31.556952 Gs [note 2] [note 3] | |
Milliday | md | ≡ 1⁄1000 d | = 86.4 s |
Minute | min | ≡ 60 s, due to leap seconds sometimes 59 s or 61 s, | = 60 s [note 3] |
Moment | ≡ 90 s | = 90 s | |
Month (full) | mo | ≡ 30 d [26] | = 2.592×106 s [note 3] |
Month (Greg. av.) | mo | = 30.436875 d | ≈ 2.6297 Ms [note 3] |
Month (hollow) | mo | ≡ 29 d [26] | = 2.5056 Ms [note 3] |
Month (synodic) | mo | Cycle time of moon phases ≈ 29.530589 d (average) | ≈ 2.551 Ms |
Octaeteris | = 48 mo (full) + 48 mo (hollow) + 3 mo (full) [27] [28] = 8 a of 365.25 d = 2922 d | = 252.4608 Ms [note 3] | |
Planck time | ≡ ( G ℏ ⁄ c 5)1⁄2 | ≈ 5.39116×10−44 s [29] | |
Second (SI base unit) | s | ≡ Time of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at 0 K [12] (but other seconds are sometimes used in astronomy). Also that time it takes for light to travel a distance of 299792458 metres. | (SI base unit) |
Shake | ≡ 10−8 s | = 10 ns | |
Sigma | ≡ 10−6 s | = 1 μs | |
Sothic cycle | ≡ 1461 a of 365 d | = 46.074096 Gs | |
Svedberg | S | ≡ 10−13 s | = 100 fs |
Week | wk | ≡ 7 d = 168 h = 10080 min | = 604.8 ks [note 3] |
Year (common) | a, y, or yr | 365 d | = 31.536 Ms [note 3] [30] |
Year (Gregorian) | a, y, or yr | = 365.2425 d average, calculated from common years (365 d) plus leap years (366 d) on most years divisible by 4. See leap year for details. | = 31.556952 Ms [note 3] |
Year (Julian) | a, y, or yr | = 365.25 d average, calculated from common years (365 d) plus one leap year (366 d) every four years | = 31.5576 Ms |
Year (leap) | a, y, or yr | 366 d | = 31.6224 Ms [note 3] [30] |
Year (mean tropical) | a, y, or yr | Conceptually, the length of time it takes for the Sun to return to the same position in the cycle of seasons, [Converter 1] approximately 365.24219 d, each day being 86400 SI seconds [31] | ≈ 31.556925 Ms |
Year (sidereal) | a, y, or yr | ≡ Time taken for Sun to return to the same position with respect to the stars of the celestial sphere, approximately 365.256363 d | ≈ 31.5581497632 Ms |
Notes:
|
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
Actions per minute | APM | ≡ 1/60 Hz | ≈ 0.0167 Hz |
Cycle per second | cps | ≡ 1 Hz | = 1 cps = 1 Hz |
degree per second | deg/s | ≡ 1 °/s ≡ 1/360 Hz | = 0.0027 Hz |
hertz (SI unit) | Hz | ≡ One cycle per second | = 1 Hz = 1/s |
Radian per second | rad/s | ≡ 1/(2π) Hz | ≈ 0.159155 Hz |
revolutions per minute | rpm | ≡ One unit rpm equals one rotation completed around a fixed axis in one minute of time. | ≈ 0.104719755 rad/s |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
foot per hour | fph | ≡ 1 ft/h | = 8.46×10−5 m/s |
foot per minute | fpm | ≡ 1 ft/min | = 5.08×10−3 m/s |
foot per second | fps | ≡ 1 ft/s | = 3.048×10−1 m/s |
furlong per fortnight | ≡ furlong/fortnight | ≈ 1.663095×10−4 m/s | |
inch per hour | iph | ≡ 1 in/h | = 7.05×10−6 m/s |
inch per minute | ipm | ≡ 1 in/min | = 4.23×10−4 m/s |
inch per second | ips | ≡ 1 in/s | = 2.54×10−2 m/s |
kilometre per hour | km/h | ≡ 1 km/h | = 2.7×10−1 m/s |
knot | kn | ≡ 1 nmi/h = 1.852 km/h | = 0.514 m/s |
knot (Admiralty) | kn | ≡ 1 NM (Adm)/h = 1.853184 km/h [32] | = 0.514773 m/s |
mach number | M | Ratio of the speed to the speed of sound [note 1] in the medium (unitless). | ≈ 340 m/s in air at sea level ≈ 295 m/s in air at jet altitudes |
metre per second (SI unit) | m/s | ≡ 1 m/s | = 1 m/s |
mile per hour | mph | ≡ 1 mi/h | = 0.44704 m/s |
mile per minute | mpm | ≡ 1 mi/min | = 26.8224 m/s |
mile per second | mps | ≡ 1 mi/s | = 1609.344 m/s |
speed of light in vacuum | c | ≡ 299792458 m/s | = 299792458 m/s |
speed of sound in air | s | 1225 to 1062 km/h (761–660 mph or 661–574 kn) [note 1] | ≈ 340 to 295 m/s |
|
A velocity consists of a speed combined with a direction; the speed part of the velocity takes units of speed.
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
cubic foot per minute | CFM[ citation needed ] | ≡ 1 ft3/min | = 4.719474432×10−4 m3/s |
cubic foot per second | ft3/s | ≡ 1 ft3/s | = 0.028316846592 m3/s |
cubic inch per minute | in3/min | ≡ 1 in3/min | = 2.7311773×10−7 m3/s |
cubic inch per second | in3/s | ≡ 1 in3/s | = 1.6387064×10−5 m3/s |
cubic metre per second (SI unit) | m3/s | ≡ 1 m3/s | = 1 m3/s |
gallon (US fluid) per day | GPD[ citation needed ] | ≡ 1 gal/d | = 4.381263638×10−8 m3/s |
gallon (US fluid) per hour | GPH[ citation needed ] | ≡ 1 gal/h | = 1.051503273×10−6 m3/s |
gallon (US fluid) per minute | GPM[ citation needed ] | ≡ 1 gal/min | = 6.30901964×10−5 m3/s |
litre per minute | l/min or L/min | ≡ 1 L/min | = 1.6×10−5 m3/s |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
foot per hour per second | fph/s | ≡ 1 ft/(h⋅s) | = 8.46×10−5 m/s2 |
foot per minute per second | fpm/s | ≡ 1 ft/(min⋅s) | = 5.08×10−3 m/s2 |
foot per second squared | fps2 | ≡ 1 ft/s2 | = 3.048×10−1 m/s2 |
gal; galileo | Gal | ≡ 1 cm/s2 | = 10−2 m/s2 |
inch per minute per second | ipm/s | ≡ 1 in/(min⋅s) | = 4.23×10−4 m/s2 |
inch per second squared | ips2 | ≡ 1 in/s2 | = 2.54×10−2 m/s2 |
knot per second | kn/s | ≡ 1 kn/s | ≈ 5.14×10−1 m/s2 |
metre per second squared (SI unit) | m/s2 | ≡ 1 m/s2 | = 1 m/s2 |
mile per hour per second | mph/s | ≡ 1 mi/(h⋅s) | = 4.4704×10−1 m/s2 |
mile per minute per second | mpm/s | ≡ 1 mi/(min⋅s) | = 26.8224 m/s2 |
mile per second squared | mps2 | ≡ 1 mi/s2 | = 1.609344×103 m/s2 |
standard gravity | ɡ0 | ≡ 9.80665 m/s2 | = 9.80665 m/s2 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atomic unit of force | ≡ me⋅α 2⋅c 2⁄ a0 | ≈ 8.23872206×10−8 N [34] | |
dyne (cgs unit) | dyn | ≡ g⋅cm/s2 | = 10−5 N |
kilogram-force; kilopond; grave-force | kgf; kp; Gf | ≡ ɡ0 × 1 kg | = 9.80665 N |
kip; kip-force | kip; kipf; klbf | ≡ ɡ0 × 1000 lb | = 4.4482216152605×103 N |
milligrave-force, gravet-force | mGf; gf | ≡ ɡ0 × 1 g | = 9.80665 mN |
long ton-force | tnf[ citation needed ] | ≡ ɡ0 × 1 long ton | = 9.96401641818352×103 N |
newton (SI unit) | N | A force capable of giving a mass of one kilogram an acceleration of one metre per second per second. [35] | = 1 N = 1 kg⋅m/s2 |
ounce-force | ozf | ≡ ɡ0 × 1 oz | = 0.27801385095378125 N |
pound-force | lbf | ≡ ɡ0 × 1 lb | = 4.4482216152605 N |
poundal | pdl | ≡ 1 lb⋅ft/s2 | = 0.138254954376 N |
short ton-force | tnf[ citation needed ] | ≡ ɡ0 × 1 short ton | = 8.896443230521×103 N |
sthene (mts unit) | sn | ≡ 1 t⋅m/s2 | = 103 N |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atmosphere (standard) | atm | ≡ 101325 Pa [36] | |
atmosphere (technical) | at | ≡ 1 kgf/cm2 | = 9.80665×104 Pa [36] |
bar | bar | ≡ 100000 Pa | ≡ 105 Pa |
barye (cgs unit) | ≡ 1 dyn/cm2 | = 0.1 Pa | |
centimetre of mercury | cmHg | ≡ 13595.1 kg/m3 × 1 cm × ɡ0 | ≈ 1.33322×103 Pa [36] |
centimetre of water (4 °C) | cmH2O | ≈ 999.972 kg/m3 × 1 cm × ɡ0 | ≈ 98.0638 Pa [36] |
foot of mercury (conventional) | ftHg | ≡ 13595.1 kg/m3 × 1 ft × ɡ0 | ≈ 4.063666×104 Pa [36] |
foot of water (39.2 °F) | ftH2O | ≈ 999.972 kg/m3 × 1 ft × ɡ0 | ≈ 2.98898×103 Pa [36] |
inch of mercury (conventional) | inHg | ≡ 13595.1 kg/m3 × 1 in × ɡ0 | ≈ 3.386389×103 Pa [36] |
inch of water (39.2 °F) | inH2O | ≈ 999.972 kg/m3 × 1 in × ɡ0 | ≈ 249.082 Pa [36] |
kilogram-force per square millimetre | kgf/mm2 | ≡ 1 kgf/mm2 | = 9.80665×106 Pa [36] |
kip per square inch | ksi | ≡ 1 kipf/sq in | ≈ 6.894757×106 Pa [36] |
long ton per square foot | ≡ 1 long ton × ɡ0 / 1 sq ft | ≈ 1.0725178011595×105 Pa | |
micrometre of mercury | μmHg | ≡ 13595.1 kg/m3 × 1 μm × ɡ0 ≈ 0.001 torr | ≈ 0.1333224 Pa [36] |
millimetre of mercury | mmHg | ≡ 13595.1 kg/m3 × 1 mm × ɡ0 ≈ 1 torr | ≈ 133.3224 Pa [36] |
millimetre of water (3.98 °C) | mmH2O | ≈ 999.972 kg/m3 × 1 mm × ɡ0 = 0.999972 kgf/m2 | = 9.80638 Pa |
pascal (SI unit) | Pa | ≡ N/m2 = kg/(m⋅s2) | = 1 Pa [37] |
pièze (mts unit) | pz | ≡ 1000 kg/m⋅s2 | = 103 Pa = 1 kPa |
pound per square foot | psf | ≡ 1 lbf/ft2 | ≈ 47.88026 Pa [36] |
pound per square inch | psi | ≡ 1 lbf/in2 | ≈ 6.894757×103 Pa [36] |
poundal per square foot | pdl/sq ft | ≡ 1 pdl/sq ft | ≈ 1.488164 Pa [36] |
short ton per square foot | ≡ 1 short ton × ɡ0 / 1 sq ft | ≈ 9.5760518×104 Pa | |
torr | torr | ≡ 101325⁄760 Pa | ≈ 133.3224 Pa [36] |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
pound-force-foot | lbf•ft | ≡ ɡ0 × 1 lb × 1 ft | = 1.3558179483314004 N⋅m |
poundal-ft | pdl•ft | ≡ 1 lb⋅ft2/s2 | = 4.21401100938048×10−2 N⋅m |
pound force-inch | lbf•in | ≡ ɡ0 × 1 lb × 1 in | = 0.1129848290276167 N⋅m |
kilogram force-meter | kgf•m | ≡ ɡ0 × N × m | = 9.80665 N⋅m |
Newton metre (SI unit) | N⋅m | ≡ N × m = kg⋅m2/s2 | = 1 N⋅m |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
barrel of oil equivalent | boe | ≈ 5.8×106 BTU59 °F | ≈ 6.12×109 J |
British thermal unit (ISO) | BTUISO | ≡ 1.0545×103 J | = 1.0545×103 J |
British thermal unit (International Table) | BTUIT | = 1.05505585262×103 J | |
British thermal unit (mean) | BTUmean | ≈ 1.05587×103 J | |
British thermal unit (thermochemical) | BTUth | ≈ 1.054350×103 J | |
British thermal unit (39 °F) | BTU39 °F | ≈ 1.05967×103 J | |
British thermal unit (59 °F) | BTU59 °F | ≡ 1.054804×103 J | = 1.054804×103 J |
British thermal unit (60 °F) | BTU60 °F | ≈ 1.05468×103 J | |
British thermal unit (63 °F) | BTU63 °F | ≈ 1.0546×103 J | |
calorie (International Table) | calIT | ≡ 4.1868 J | = 4.1868 J |
calorie (mean) | calmean | 1⁄100 of the energy required to warm one gram of air-free water from 0 °C to 100 °C at a pressure of 1 atm | ≈ 4.19002 J |
calorie (thermochemical) | calth | ≡ 4.184 J | = 4.184 J |
Calorie (US; FDA) | Cal | ≡ 1 kcal = 1000 cal | = 4184 J |
calorie (3.98 °C) | cal3.98 °C | ≈ 4.2045 J | |
calorie (15 °C) | cal15 °C | ≡ 4.1855 J | = 4.1855 J |
calorie (20 °C) | cal20 °C | ≈ 4.1819 J | |
Celsius heat unit (International Table) | CHUIT | ≡ 1 BTUIT × 1 K/°R | = 1.899100534716×103 J |
cubic centimetre of atmosphere; standard cubic centimetre | cc atm; scc | ≡ 1 atm × 1 cm3 | = 0.101325 J |
cubic foot of atmosphere; standard cubic foot | cu ft atm; scf | ≡ 1 atm × 1 ft3 | = 2.8692044809344×103 J |
cubic foot of natural gas | ≡ 1000 BTUIT | = 1.05505585262×106 J | |
cubic yard of atmosphere; standard cubic yard | cu yd atm; scy | ≡ 1 atm × 1 yd3 | = 77.4685209852288×103 J |
electronvolt | eV | ≡ e × 1 V | ≈ 1.602176565(35)×10−19 J |
erg (cgs unit) | erg | ≡ 1 g⋅cm2/s2 | = 10−7 J |
foot-pound force | ft lbf | ≡ ɡ0 × 1 lb × 1 ft | = 1.3558179483314004 J |
foot-poundal | ft pdl | ≡ 1 lb⋅ft2/s2 | = 4.21401100938048×10−2 J |
gallon-atmosphere (imperial) | imp gal atm | ≡ 1 atm × 1 gal (imp) | = 460.63256925 J |
gallon-atmosphere (US) | US gal atm | ≡ 1 atm × 1 gal (US) | = 383.5568490138 J |
hartree, atomic unit of energy | Eh | ≡ me⋅α 2⋅c 2 (= 2 Ry) | ≈ 4.359744×10−18 J |
horsepower-hour | hp⋅h | ≡ 1 hp × 1 h | = 2.684519537696172792×106 J |
inch-pound force | in lbf | ≡ ɡ0 × 1 lb × 1 in | = 0.1129848290276167 J |
joule (SI unit) | J | The work done when a force of one newton moves the point of its application a distance of one metre in the direction of the force. [35] | = 1 J = 1 m⋅N = 1 kg⋅m2/s2 = 1 C⋅V = 1 W⋅s |
kilocalorie; large calorie | kcal; Cal | ≡ 1000 calIT | = 4.1868×103 J |
kilowatt-hour; Board of Trade Unit | kW⋅h; B.O.T.U. | ≡ 1 kW × 1 h | = 3.6×106 J |
litre-atmosphere | l atm; sl | ≡ 1 atm × 1 L | = 101.325 J |
quad | ≡ 1015 BTUIT | = 1.05505585262×1018 J | |
rydberg | Ry | ≡ R ∞⋅ℎ⋅c | ≈ 2.179872×10−18 J |
therm (E.C.) | ≡ 100000 BTUIT | = 105.505585262×106 J | |
therm (US) | ≡ 100000 BTU59 °F | = 105.4804×106 J | |
thermie | th | ≡ 1 McalIT | = 4.1868×106 J |
tonne of coal equivalent | TCE | ≡ 7 Gcalth | = 29.288×109 J |
tonne of oil equivalent | toe | ≡ 10 GcalIT | = 41.868×109 J |
ton of TNT | tTNT | ≡ 1 Gcalth | = 4.184×109 J |
watt hour | W⋅h | ≡ 1 W × 1 h | = 3.6×103 J |
watt second | W⋅s | ≡ 1 J | = 1×100 J |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atmosphere-cubic centimetre per minute | atm ccm[ citation needed ] | ≡ 1 atm × 1 cm3/min | = 1.68875×10−3 W |
atmosphere-cubic centimetre per second | atm ccs[ citation needed ] | ≡ 1 atm × 1 cm3/s | = 0.101325 W |
atmosphere-cubic foot per hour | atm cfh[ citation needed ] | ≡ 1 atm × 1 cu ft/h | = 0.79700124704 W |
atmosphere-cubic foot per minute | atm cfm[ citation needed ] | ≡ 1 atm × 1 cu ft/min | = 47.82007468224 W |
atmosphere-cubic foot per second | atm cfs[ citation needed ] | ≡ 1 atm × 1 cu ft/s | = 2.8692044809344×103 W |
BTU (International Table) per hour | BTUIT/h | ≡ 1 BTUIT/h | ≈ 0.293071 W |
BTU (International Table) per minute | BTUIT/min | ≡ 1 BTUIT/min | ≈ 17.584264 W |
BTU (International Table) per second | BTUIT/s | ≡ 1 BTUIT/s | = 1.05505585262×103 W |
calorie (International Table) per second | calIT/s | ≡ 1 calIT/s | = 4.1868 W |
erg per second | erg/s | ≡ 1 erg/s | = 10−7 W |
foot-pound-force per hour | ft⋅lbf/h | ≡ 1 ft lbf/h | ≈ 3.766161×10−4 W |
foot-pound-force per minute | ft⋅lbf/min | ≡ 1 ft lbf/min | = 2.259696580552334×10−2 W |
foot-pound-force per second | ft⋅lbf/s | ≡ 1 ft lbf/s | = 1.3558179483314004 W |
horsepower (boiler) | hp | ≈ 34.5 lb/h × 970.3 BTUIT/lb | ≈ 9809.5 W [38] |
horsepower (European electrical) | hp | ≡ 75 kp⋅m/s | = 736 W[ citation needed ] |
horsepower (electrical) | hp | ≡ 746 W | = 746 W [38] |
horsepower (mechanical) | hp | ≡ 550 ft⋅lbf/s [38] | = 745.69987158227022 W |
horsepower (metric) | hp or PS | ≡ 75 m⋅kgf/s | = 735.49875 W [38] |
litre-atmosphere per minute | L·atm/min | ≡ 1 atm × 1 L/min | = 1.68875 W |
litre-atmosphere per second | L·atm/s | ≡ 1 atm × 1 L/s | = 101.325 W |
lusec | lusec | ≡ 1 L·µmHg/s [20] | ≈ 1.333×10−4 W |
poncelet | p | ≡ 100 m⋅kgf/s | = 980.665 W |
square foot equivalent direct radiation | sq ft EDR | ≡ 240 BTUIT/h | ≈ 70.337057 W |
ton of air conditioning | ≡ 2000 lb of ice melted / 24 h | ≈ 3504 W | |
ton of refrigeration (imperial) | ≡ 2240 lb × iceIT / 24 h: iceIT = 144 °F × 2326 J/kg⋅°F | ≈ 3.938875×103 W | |
ton of refrigeration (IT) | ≡ 2000 lb × iceIT / 24 h: iceIT = 144 °F × 2326 J/kg⋅°F | ≈ 3.516853×103 W | |
watt (SI unit) | W | The power which in one second of time gives rise to one joule of energy. [35] | = 1 W = 1 J/s = 1 N⋅m/s = 1 kg⋅m2/s3 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atomic unit of action | au | ≡ ℏ ≡ ℎ ⁄2π | ≈ 1.05457168×10−34 J⋅s [39] |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
pascal second (SI unit) | Pa⋅s | ≡ N⋅s/m2, kg/(m⋅s) | = 1 Pa⋅s |
poise (cgs unit) | P | ≡ 1 barye⋅s | = 0.1 Pa⋅s |
pound per foot hour | lb/(ft⋅h) | ≡ 1 lb/(ft⋅h) | ≈ 4.133789×10−4 Pa⋅s |
pound per foot second | lb/(ft⋅s) | ≡ 1 lb/(ft⋅s) | ≈ 1.488164 Pa⋅s |
pound-force second per square foot | lbf⋅s/ft2 | ≡ 1 lbf⋅s/ft2 | ≈ 47.88026 Pa⋅s |
pound-force second per square inch | lbf⋅s/in2 | ≡ 1 lbf⋅s/in2 | ≈ 6894.757 Pa⋅s |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
square foot per second | ft2/s | ≡ 1 ft2/s | = 0.09290304 m2/s |
square metre per second (SI unit) | m2/s | ≡ 1 m2/s | = 1 m2/s |
stokes (cgs unit) | St | ≡ 1 cm2/s | = 10−4 m2/s |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
ampere (SI base unit) | A | ≡ one coulomb of charge going past a given point per second. [40] | (SI base unit) |
electromagnetic unit; abampere (cgs unit) | abamp | ≡ 10 A | = 10 A |
esu per second; statampere (cgs unit) | esu/s | ≡ 0.1 A⋅m/s⁄ c | ≈ 3.335641×10−10 A |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
abcoulomb; electromagnetic unit (cgs unit) | abC; emu | ≡ 10 C | = 10 C |
atomic unit of charge | au | ≡ e | ≈ 1.602176462×10−19 C |
coulomb | C | ≡ charge of exactly 1/(1.602176634×10−19) elementary charges [40] | = 1 C = 1 A⋅s |
faraday | F | ≡ 1 mol × NA⋅e | ≈ 96485.3383 C |
milliampere hour | mA⋅h | ≡ 0.001 A × 1 h | = 3.6 C |
statcoulomb; franklin; electrostatic unit (cgs unit) | statC; Fr; esu | ≡ 0.1 A⋅m⁄ c | ≈ 3.335641×10−10 C |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
atomic unit of electric dipole moment | e a0 | ≈ 8.47835281×10−30 C⋅m [41] | |
coulomb meter | C⋅m | = 1 C × 1 m | |
debye | D | = 10−10 esu⋅Å | = 3.33564095×10−30 C⋅m [42] |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
abvolt (cgs unit) | abV | ≡ 10−8 V | = 10−8 V |
statvolt (cgs unit) | statV | ≡ c⋅(1 μJ/A⋅m) | = 299.792458 V |
volt (SI unit) | V | The difference in electric potential across two points along a conducting wire carrying one ampere of constant current when the power dissipated between the points equals one watt. [35] | = 1 V = 1 W/A = 1 kg⋅m2/(A⋅s3) = 1 J/C |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
ohm (SI unit) | Ω | The resistance between two points in a conductor when one volt of electric potential difference, applied to these points, produces one ampere of current in the conductor. [35] | = 1 Ω = 1 V/A = 1 kg⋅m2/(A2⋅s3) |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
farad (SI unit) | F | The capacitance between two parallel plates that results in one volt of potential difference when charged by one coulomb of electricity. [35] | = 1 F = 1 C/V = 1 A2⋅s4/(kg⋅m2) |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
maxwell (CGS unit) | Mx | ≡ 10−8 Wb [38] | = 10−8 Wb |
weber (SI unit) | Wb | Magnetic flux which, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second. [35] | = 1 Wb = 1 V⋅s = 1 kg⋅m2/(A⋅s2) |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
gauss (CGS unit) | G | ≡ Mx/cm2 = 10−4 T | = 10−4 T [43] |
tesla (SI unit) | T | ≡ Wb/m2 | = 1 T = 1 Wb/m2= 1 kg/(A⋅s2) |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
henry (SI unit) | H | The inductance of a closed circuit that produces one volt of electromotive force when the current in the circuit varies at a uniform rate of one ampere per second. [35] | = 1 H = 1 Wb/A = 1 kg⋅m2/(A⋅s)2 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
degree Celsius | °C | [°C] ≡ [K] − 273.15 | [K] ≡ [°C] + 273.15 |
degree Delisle | °De | [K] = 373.15 − [°De] × 2⁄3 | |
degree Fahrenheit | °F | [°F] ≡ [°C] × 9⁄5 + 32 | [K] ≡ ([°F] + 459.67) × 5⁄9 |
degree Newton | °N | [K] = [°N] × 100⁄33 + 273.15 | |
degree Rankine | °R; | [°R] ≡ [K] × 9⁄5 | [K] ≡ [°R] × 5/9 |
degree Réaumur | °Ré | [K] = [°Ré] × 5⁄4 + 273.15 | |
degree Rømer | °Rø | [K] = ([°Rø] − 7.5) × 40⁄21 + 273.15 | |
Regulo Gas Mark | GM | [°F] ≡ [GM] × 25 + 300 | [K] ≡ [GM] × 125⁄9 + 422.038 |
kelvin (SI base unit) | K | ≡ change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380 649×10−23 J. [44] | (SI base unit) |
Name of unit | Symbol | Definition | Relation to SI units | Relation to bits |
---|---|---|---|---|
natural unit of information; nip; nepit | nat | |||
shannon; bit | Sh; bit; b | ≡ ln(2) × nat | ≈ 0.693147 nat | = 1 bit |
hartley; ban | Hart; ban | ≡ ln(10) × nat | ≈ 2.302585 nat | |
nibble | ≡ 4 bits | = 22 bit | ||
byte | B | ≡ 8 bits | = 23 bit | |
kilobyte (decimal) | kB | ≡ 1000 B | = 8000 bit | |
kilobyte (kibibyte) | KB; KiB | ≡ 1024 B | = 213 bit = 8192 bit |
Modern standards (such as ISO 80000) prefer the shannon to the bit as a unit for a quantity of information entropy, whereas the (discrete) storage space of digital devices is measured in bits. Thus, uncompressed redundant data occupy more than one bit of storage per shannon of information entropy. The multiples of a bit listed above are usually used with this meaning.
The candela is the preferred nomenclature for the SI unit.
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
candela (SI base unit); candle | cd | The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. [40] | (SI base unit) |
candlepower (new) | cp | ≡ cd The use of candlepower as a unit is discouraged due to its ambiguity. | = 1 cd |
candlepower (old, pre-1948) | cp | Varies and is poorly reproducible. [45] Approximately 0.981 cd. [20] | ≈ 0.981 cd |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
candela per square foot | cd/ft2 | ≡ cd/ft2 | ≈ 10.763910417 cd/m2 |
candela per square inch | cd/in2 | ≡ cd/in2 | ≈ 1550.0031 cd/m2 |
candela per square metre (SI unit); nit (deprecated [20] ) | cd/m2 | ≡ cd/m2 | = 1 cd/m2 |
footlambert | fL | ≡ (1/π) cd/ft2 | ≈ 3.4262590996 cd/m2 |
lambert | L | ≡ (104/π) cd/m2 | ≈ 3183.0988618 cd/m2 |
stilb (CGS unit) | sb | ≡ 104 cd/m2 | = 104 cd/m2 |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
lumen (SI unit) | lm | The luminous flux of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant flux of 1/683 watt. [40] | = 1 lm = 1 cd⋅sr |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
footcandle; lumen per square foot | fc | ≡ lm/ft2 | = 10.763910417 lx |
lumen per square inch | lm/in2 | ≡ lm/in2 | ≈ 1550.0031 lx |
lux (SI unit) | lx | ≡ lm/m2 | = 1 lx = 1 lm/m2 |
phot (CGS unit) | ph | ≡ lm/cm2 | = 104 lx |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
becquerel (SI unit) | Bq | ≡ Number of disintegrations per second | = 1 Bq = 1/s |
curie | Ci | ≡ 3.7×1010 Bq [46] | = 3.7×1010 Bq |
rutherford (H) | Rd | ≡ 1 MBq | = 106 Bq |
Although becquerel (Bq) and hertz (Hz) both ultimately refer to the same SI base unit (s−1), Hz is used only for periodic phenomena (i.e. repetitions at regular intervals), and Bq is only used for stochastic processes (i.e. at random intervals) associated with radioactivity. [47]
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
roentgen | R | 1 R ≡ 2.58×10−4 C/kg [38] | = 2.58×10−4 C/kg |
The roentgen is not an SI unit and the NIST strongly discourages its continued use. [48]
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
gray (SI unit) | Gy | ≡ 1 J/kg = 1 m2/s2 [49] | = 1 Gy = 1 J/kg = 1 m2s2 |
rad | rad | ≡ 0.01 Gy [38] | = 0.01 Gy |
Name of unit | Symbol | Definition | Relation to SI units |
---|---|---|---|
Röntgen equivalent man | rem | ≡ 0.01 Sv | = 0.01 Sv |
sievert (SI unit) | Sv | ≡ 1 J/kg [47] | = 1 Sv = 1 J/kg = 1 m2s2 |
Although the definitions for sievert (Sv) and gray (Gy) would seem to indicate that they measure the same quantities, this is not the case. The effect of receiving a certain dose of radiation (given as Gy) is variable and depends on many factors, thus a new unit was needed to denote the biological effectiveness of that dose on the body; this is known as the equivalent dose and is shown in Sv. The general relationship between absorbed dose and equivalent dose can be represented as
where H is the equivalent dose, D is the absorbed dose, and Q is a dimensionless quality factor. Thus, for any quantity of D measured in Gy, the numerical value for H measured in Sv may be different. [50]
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The candela is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function. A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.
The centimetre–gram–second system of units is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.
The decibel is a relative unit of measurement corresponding to one tenth of a bel (B). It is used to express the ratio of one value of a power or root-power quantity to another, on a logarithmic scale. A logarithmic quantity in decibels is called a level. Two signals whose levels differ by one decibel have a power ratio of 101/10 or an amplitude ratio of 101⁄20.
The litre or liter is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3). A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to one-thousandth of a cubic metre.
The International System of Units is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second, metre, kilogram, ampere, kelvin, mole, and candela. The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units. Twenty-two derived units have been provided with special names and symbols. The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. The SI system also provides twenty prefixes to the unit names and unit symbols that may be used when specifying power-of-ten multiples and sub-multiples of SI units. The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
The neper is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and bel, the neper is a unit defined in the international standard ISO 80000. It is not part of the International System of Units (SI), but is accepted for use alongside the SI.
The Avogadro constant (NA or L) is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. Its SI unit is the reciprocal mole, and it is defined as NA = 6.02214076×1023 mol−1. It is named after the Italian scientist Amedeo Avogadro.Although this is called Avogadro's constant (or number), he is not the chemist who determined its value. Stanislao Cannizzarro explained this number four years after Avogadro's death while at the Karlsruhe Congress in 1860.
The dalton or unified atomic mass unit is a unit of mass widely used in physics and chemistry. It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The atomic mass constant, denoted mu is defined identically, giving mu = m(12C)/12 = 1 Da.
The becquerel is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relating to human health this is a small quantity, and SI multiples of the unit are commonly used.
Gaussian units constitute a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs (centimetre–gram–second) units. It is also called the Gaussian unit system, Gaussian-cgs units, or often just cgs units. The term "cgs units" is ambiguous and therefore to be avoided if possible: there are several variants of cgs with conflicting definitions of electromagnetic quantities and units.
ISO 31-0 is the introductory part of international standard ISO 31 on quantities and units. It provides guidelines for using physical quantities, quantity and unit symbols, and coherent unit systems, especially the SI. It is intended for use in all fields of science and technology and is augmented by more specialized conventions defined in other parts of the ISO 31 standard. ISO 31-0 was withdrawn on 17 November 2009. It is superseded by ISO 80000-1. Other parts of ISO 31 have also been withdrawn and replaced by parts of ISO 80000.
Vacuum permittivity, commonly denoted ε0 is the value of the absolute dielectric permittivity of classical vacuum. Alternatively it may be referred to as the permittivity of free space, the electric constant, or the distributed capacitance of the vacuum. It is an ideal (baseline) physical constant. Its CODATA value is:
Lorentz–Heaviside units constitute a system of units within CGS, named for Hendrik Antoon Lorentz and Oliver Heaviside. They share with CGS-Gaussian units the property that the electric constant ε0 and magnetic constant µ0 do not appear, having been incorporated implicitly into the electromagnetic quantities by the way they are defined. Lorentz–Heaviside units may be regarded as normalizing ε0 = 1 and µ0 = 1, while at the same time revising Maxwell's equations to use the speed of light c instead.
Vacuum permeability is the magnetic permeability in a classical vacuum. Vacuum permeability is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum. Since the redefinition of SI units in 2019, the vacuum permeability μ0 is no longer a defined constant, but rather needs to be determined experimentally.
A g-factor is a dimensionless quantity that characterizes the magnetic moment and angular momentum of an atom, a particle or the nucleus. It is essentially a proportionality constant that relates the observed magnetic moment μ of a particle to its angular momentum quantum number and a unit of magnetic moment, usually the Bohr magneton or nuclear magneton.
The Planck constant, or Planck's constant, is the quantum of electromagnetic action that relates a photon's energy to its frequency. The Planck constant multiplied by a photon's frequency is equal to a photon's energy. The Planck constant is a fundamental physical constant denoted as , and of fundamental importance in quantum mechanics. In metrology it is used to define the kilogram in SI units.
A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres, we actually mean 10 times the definite predetermined length called "metre". Measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind.
In 2019, the SI base units were redefined in agreement with the International System of Quantities, effective on the 144th anniversary of the Metre Convention, 20 May 2019. In the redefinition, four of the seven SI base units – the kilogram, ampere, kelvin, and mole – were redefined by setting exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela were already defined by physical constants and were not subject to correction to their definitions. The new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.
The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other dimensions with unity ratios were added, and the system went on to be adopted across the world.
In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of four universal physical constants, in such a manner that these physical constants take on the numerical value of 1 when expressed in terms of these units.