While the International System of Units (SI) is used throughout the world in all fields, many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The authority behind the SI system, the General Conference on Weights and Measures, recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI.^{ [1] }

- Units officially accepted for use with the SI
- Other units defined but not officially sanctioned
- Changes to units mentioned in the SI
- See also
- Notes and references
- Notes
- References

Some units of time, angle, and legacy non-SI units have a long history of use. Most societies have used the solar day and its non-decimal subdivisions as a basis of time and, unlike the foot or the pound, these were the same regardless of where they were being measured. The radian, being 1/2π of a revolution, has mathematical advantages but is rarely used for navigation. Further, the units used in navigation around the world are similar. The tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are given below.^{ [2] }

Most of these, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min = 60 s), the hour (3600 s), and the day (86400 s); the degree (for measuring plane angles, 1° = π⁄180rad); and the electronvolt (a unit of energy, 1 eV = 1.602176634×10^{−19} J).^{ [2] }

This is a list of units that are not defined as part of the International System of Units (SI) but are otherwise mentioned in the SI Brochure,^{ [2] } listed as being accepted for use alongside SI-units, or for explanatory purposes.

Name | Symbol | Quantity | Value in SI units |
---|---|---|---|

minute | min | time | 1 min = 60 s |

hour | h | time | 1 h = 60 min = 3600 s |

day | d | time | 1 d = 24 h = 1440 min = 86400 s |

astronomical unit | au | length | 1 au = 149597870700 m |

degree | ° | plane angle and phase angle | 1° = (π⁄180) rad |

arcminute | ′ | plane angle and phase angle | 1′ = (1⁄60)° = (π⁄10800) rad |

arcsecond | ″ | plane angle and phase angle | 1″ = (1⁄60)′ = (1⁄3600)° = (π⁄648000) rad |

hectare | ha | area | 1 ha = 1 hm^{2} = 10000 m^{2} |

litre | l, L | volume | 1 L = 1 dm^{3} = 1000 cm^{3} = 0.001 m^{3} |

tonne | t | mass | 1 t = 1 Mg = 1000 kg |

dalton | Da | mass | 1 Da = 1.66053906660(50)×10^{−27} kg^{ [3] }^{ [note 1] } |

electronvolt | eV | energy | 1 eV = 1.602176634×10^{−19} J^{ [4] } |

neper | Np | logarithmic ratio quantity | 1 Np = 1 |

bel, decibel | B, dB | logarithmic ratio quantity | — |

The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time.

The following table lists units that are effectively defined in sidenotes and footnotes in the 9th SI brochure. Units that are mentioned without a definition or that occur in historical material recorded in the appendices are not included.

Name | Symbol | Quantity | Equivalent official unit |
---|---|---|---|

gal ^{ [note 2] } | Gal | acceleration | 1 Gal = 1 cm/s^{2} = 0.01 m/s^{2} |

unified atomic mass unit ^{ [note 3] } | u | mass | 1 u = 1 Da |

volt-ampere reactive | var | reactive power | 1 var = 1 VA |

With the publication of each edition of the SI brochure, the list of non-SI units listed in tables changed compared to the preceding SI brochures.^{ [5] } The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.

Name | Symbol | 1st–3rd SI Brochures | 4–6th SI Brochures | 7th SI Brochure | 8th SI Brochure | 9th SI Brochure |
---|---|---|---|---|---|---|

astronomical unit | au^{ [note 4] } | accepted | omitted | accepted | accepted | accepted |

parsec | pc | accepted | omitted | omitted | omitted | omitted |

neper | Np | omitted | omitted | accepted | listed | accepted |

bel | B | omitted | omitted | accepted | listed | accepted |

decibel | dB | omitted | omitted | omitted | listed | accepted |

unified atomic mass unit | u | accepted | accepted | accepted | accepted | footnote |

dalton | Da | omitted | omitted | footnote | accepted | accepted |

natural unit (n.u.) of speed | c_{0} | omitted | omitted | omitted | listed | omitted |

n.u. of action | ħ | omitted | omitted | omitted | listed | omitted |

n.u. of mass | m_{e} | omitted | omitted | omitted | listed | omitted |

n.u. of time | ħ⁄(m_{e}c_{0}^{2}) | omitted | omitted | omitted | listed | omitted |

atomic unit (a.u.) of charge | e | omitted | omitted | omitted | listed | omitted |

a.u. of mass | m_{e} | omitted | omitted | omitted | listed | omitted |

a.u. of action | ħ | omitted | omitted | omitted | listed | omitted |

a.u. of length | a_{0} | omitted | omitted | omitted | listed | omitted |

a.u. of energy | E_{h} | omitted | omitted | omitted | listed | omitted |

a.u. of time | ħ⁄E_{h} | omitted | omitted | omitted | listed | omitted |

nautical mile | M | temporary | temporary | temporary | listed | omitted |

knot | kn | temporary | temporary | temporary | listed | omitted |

ångström | Å | temporary | temporary | temporary | listed | omitted |

are | a | temporary | temporary | temporary | omitted | omitted |

hectare | ha | temporary | temporary | temporary | accepted | accepted |

barn | b | temporary | temporary | temporary | listed | omitted |

bar | bar | temporary | temporary | temporary | listed | omitted |

standard atmosphere | atm | temporary | listed | listed | omitted | omitted |

gal | Gal | temporary | temporary | listed | listed | footnote |

curie | Ci | temporary | temporary | listed | omitted | omitted |

roentgen | R | temporary | temporary | listed | omitted | omitted |

rad | rad | temporary | temporary | listed | omitted | omitted |

rem | rem | omitted | temporary | listed | omitted | omitted |

erg | erg | listed | listed | listed | listed | omitted |

dyne | dyn | listed | listed | listed | listed | omitted |

poise | P | listed | listed | listed | listed | omitted |

stokes | st | listed | listed | listed | listed | omitted |

maxwell | Mx | listed | listed | listed | listed | omitted |

gauss | G | listed | listed | listed | listed | omitted |

ørsted | Oe | listed | listed | listed | listed | omitted |

phot | ph | listed | listed | listed | listed | omitted |

fermi | listed | listed | listed | omitted | omitted | |

metric carat | listed | listed | listed | omitted | omitted | |

torr | Torr | listed | listed | listed | omitted | omitted |

kilogram-force | kgf | listed | listed | omitted | omitted | omitted |

calorie | cal | listed | listed | listed | omitted | omitted |

micron | μ | listed | listed | listed | omitted | omitted |

x-unit | xu | listed | listed | omitted | omitted | omitted |

stilb | sb | listed | listed | omitted | omitted | omitted |

gamma (Mass) | γ | listed | listed | listed | omitted | omitted |

γ (magnetic flux density) | γ | listed | listed | omitted | omitted | omitted |

lambda | λ | listed | listed | omitted | omitted | omitted |

jansky | Jy | omitted | omitted | listed | omitted | omitted |

millimetre of mercury | mmHg | omitted | omitted | omitted | listed | omitted |

In this table, the status descriptions have the following meanings:

- "accepted": The unit is accepted for use with the SI.
- "temporary": The unit is accepted for use with the SI, but is planned to be phased out.
- "listed": The unit is defined in a table of units in the brochure but not accepted for use with the SI.
- "footnote": The unit is defined in a footnote or side note, without any mention in the main text.
- "omitted": There is no mention of the unit in the brochure, excluding historical appendices.

- ↑ A footnote in the 9th SI Brochure gives an exact definition of the dalton.
- ↑ This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
- ↑ The unified mass unit (u) is a synonym of the dalton (Da). In edition 9 (2019) of the SI Brochure, the unified mass unit is no longer listed as being accepted for use with SI units, though it notes its equivalence with the dalton in a footnote. In edition 8 (2006), both names were mentioned in parallel. In edition 7 (1998), position was the reverse of that in edition 9. Earlier editions mentioned only u.
- ↑ The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.

The **General Conference on Weights and Measures** is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre Convention through which member states act together on matters related to measurement science and measurement standards. The CGPM is made up of delegates of the governments of the member states and observers from the Associates of the CGPM. It elects the **International Committee for Weights and Measures** as the supervisory board of the BIPM to direct and supervise it.

The **kilogram** is the base unit of mass in the International System of Units (SI), having the unit symbol **kg**. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a **kilo** colloquially. It means 'one thousand grams'.

The **katal** is that catalytic activity that will raise the rate of conversion by one mole per second in a specified assay system. It is a unit of the International System of Units (SI) used for quantifying the catalytic activity of enzymes and other catalysts.

The **litre** or **liter** is a metric unit of volume. It is equal to 1 cubic decimetre (dm^{3}), 1000 cubic centimetres (cm^{3}) or 0.001 cubic metres (m^{3}). 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 **Metre Convention**, also known as the **Treaty of the Metre**, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden and Norway, Switzerland, Ottoman Empire, United States of America, and Venezuela.

The **International System of Units**, internationally known by the abbreviation **SI**, is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

A **metric prefix** is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix *kilo-*, for example, may be added to *gram* to indicate *multiplication* by one thousand: one kilogram is equal to one thousand grams. The prefix *milli-*, likewise, may be added to *metre* to indicate *division* by one thousand; one millimetre is equal to one thousandth of a metre.

The **Avogadro constant**, commonly denoted ** N_{A}** or

The **dalton** or **unified atomic mass unit** is a non-SI unit of mass 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 *m*_{u}, is defined identically, giving *m*_{u} = 1/12*m*(^{12}C) = 1 Da.

**Metrology** is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.

The **newton** is the unit of force in the International System of Units (SI). It is defined as , the force which gives a mass of 1 kilogram an acceleration of 1 metre per second per second. It is named after Isaac Newton in recognition of his work on classical mechanics, specifically his second law of motion.

The **bar** is a metric unit of pressure defined as 100,000 Pa (100 kPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea level. By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.

The **molar mass constant**, usually denoted by *M*_{u}, is a physical constant defined as one twelfth of the molar mass of carbon-12: *M*_{u} = *M*(^{12}C)/12. The molar mass of any element or compound is its relative atomic mass multiplied by the molar mass constant.

The **kelvin**, symbol **K**, is a unit of measurement for temperature. The **Kelvin scale** is an absolute scale, which is defined such that 0 K is absolute zero and a change of thermodynamic temperature T by 1 kelvin corresponds to a change of thermal energy *kT* by 1.380649×10^{−23} J. The Boltzmann constant *k* = 1.380649×10^{−23} J⋅K^{−1} was exactly defined in the 2019 redefinition of the SI base units such that the triple point of water is 273.16±0.0001 K. The kelvin is the base unit of temperature in the International System of Units (SI), used alongside its prefixed forms. It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).

The **angstrom** or **ångström** is a metric unit of length equal to 10^{−10} m; that is, one ten-billionth (US) of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874).

In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four 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 measures with unity ratios were added, and the system went on to be adopted across the world.

In physics, **natural units** are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a coherent unit of a quantity. For example, the elementary charge *e* may be used as a natural unit of electric charge, and the speed of light *c* may be used as a natural unit of speed. A purely natural system of units has all of its units defined such that each of these can be expressed as a product of powers of defining physical constants.

In physics, **monochromatic radiation** is electromagnetic radiation with a single constant frequency or wavelength. When that frequency is part of the visible spectrum the term **monochromatic light** is often used. Monochromatic light is perceived by the human eye as a *spectral color*.

- ↑ International Bureau of Weights and Measures (2006),
*The International System of Units (SI)*(PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16 - 1 2 3 Bureau international des poids et mesures, "Non-SI units that are accepted for use with the SI", in: Le Système international d'unités (SI) / The International System of Units (SI), 9th ed. (Sèvres: 2019), ISBN 9789282222720, c. 4, pp. 145–146.
- ↑ "2018 CODATA Value: atomic mass constant".
*The NIST Reference on Constants, Units, and Uncertainty*. NIST. 20 May 2019. Retrieved 2019-05-20. - ↑ "2018 CODATA Value: electron volt".
*The NIST Reference on Constants, Units, and Uncertainty*. NIST. 20 May 2019. Retrieved 2019-05-20. - ↑ Bureau international des poids et mesures, Le Système international d'unités (SI) / The International System of Units (SI), 8th ed. (Sèvres: Organisation Intergouvernementale de la Convention du Mètre, 2006‑05), ISBN 9282222136.

This page is based on this Wikipedia article

Text is available under the CC BY-SA 4.0 license; additional terms may apply.

Images, videos and audio are available under their respective licenses.

Text is available under the CC BY-SA 4.0 license; additional terms may apply.

Images, videos and audio are available under their respective licenses.