Non-SI units mentioned in the SI

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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, [1] listed as being accepted for use alongside SI-units, or for explanatory purposes.


Units officially accepted for use with the SI

NameSymbolQuantityValue in SI units
minute min time 1 min = 60 s
hour htime1 h = 60 min = 3 600 s
day dtime1 d = 24 h = 1440 min = 86 400 s
astronomical unit au length 1 au = 149 597 870 700 m
degree ° plane angle 1° = (π/180) rad
arcminute plane angle1′ = (1/60)° = (π/10 800) rad
arcsecond plane angle1″ = (1/60)′ = (1/3 600)° = (π/648 000) rad
hectare ha area 1 ha = 1 hm2 = 10 000 m2
litre l, L volume 1 l = 1 dm3 = 1 000 cm3 = 0.001 m3
tonne t mass 1 t = 1 Mg = 1 000 kg
dalton Damass1 Da = 1.66053906660(50)×10−27 kg [2] [note 1]
electronvolt eV energy 1 eV = 1.602176634×10−19  J
neper Nplogarithmic ratio quantity
bel, decibel B, dBlogarithmic ratio quantity

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

Other units defined but not officially sanctioned

The following table lists units that are effectively defined in side- 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.

NameSymbolQuantityEquivalent SI unit
gal [note 2] Gal acceleration 1 Gal = 1 cm⋅s−2 = 0.01  m⋅s−2
unified atomic mass unit [note 3] umass1 u = 1  Da
volt-ampere reactive varreactive power1 var = 1 V⋅A

Changes to units mentioned in the SI

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. [3] The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.

NameSymbol1st–3rd SI Brochures4–6th SI Brochures7th SI Brochure8th SI Brochure9th SI Brochure
astronomical unit au [note 4] acceptednot mentionedacceptedacceptedaccepted
parsec pcacceptednot mentionednot mentionednot mentionednot mentioned
neper Npnot mentionednot mentionedacceptedlistedaccepted
bel bnot mentionednot mentionedacceptedlistedaccepted
decibel dbnot mentionednot mentionednot mentionedlistedaccepted
unified atomic mass unit [note 3] uacceptedacceptedacceptedacceptedfootnote only
dalton [note 3] Danot mentionednot mentionedfootnote onlyacceptedaccepted
natural unit (n.u.) of speedc0not mentionednot mentionednot mentionedlistednot mentioned
n.u. of actionħnot mentionednot mentionednot mentionedlistednot mentioned
n.u. of massmenot mentionednot mentionednot mentionedlistednot mentioned
n.u. of timeħ/(mec02)not mentionednot mentionednot mentionedlistednot mentioned
atomic unit (a.u.) of chargeenot mentionednot mentionednot mentionedlistednot mentioned
a.u. of massmenot mentionednot mentionednot mentionedlistednot mentioned
a.u. of actionħnot mentionednot mentionednot mentionedlistednot mentioned
a.u. of lengtha0not mentionednot mentionednot mentionedlistednot mentioned
a.u. of energyEhnot mentionednot mentionednot mentionedlistednot mentioned
a.u. of timeħ/Ehnot mentionednot mentionednot mentionedlistednot mentioned
nautical mile Mtemporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
knot kntemporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
ångström Åtemporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
are atemporarily acceptedtemporarily acceptedtemporarily acceptednot mentionednot mentioned
hectare hatemporarily acceptedtemporarily acceptedtemporarily acceptedacceptedaccepted
barn btemporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
bar bartemporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
standard atmosphere atmtemporarily acceptedlistedlistednot mentionednot mentioned
gal Galtemporarily acceptedtemporarily acceptedlistedlistedfootnote only
curie Citemporarily acceptedtemporarily acceptedlistednot mentionednot mentioned
roentgen Rtemporarily acceptedtemporarily acceptedlistednot mentionednot mentioned
rad radtemporarily acceptedtemporarily acceptedlistednot mentionednot mentioned
rem remnot mentionedtemporarily acceptedlistednot mentionednot mentioned
erg erglistedlistedlistedlistednot mentioned
dyne dynlistedlistedlistedlistednot mentioned
poise Plistedlistedlistedlistednot mentioned
stokes stlistedlistedlistedlistednot mentioned
maxwell Mxlistedlistedlistedlistednot mentioned
gauss Glistedlistedlistedlistednot mentioned
ørsted Oelistedlistedlistedlistednot mentioned
phot phlistedlistedlistedlistednot mentioned
fermi listedlistedlistednot mentionednot mentioned
metric carat listedlistedlistednot mentionednot mentioned
torr Torrlistedlistedlistednot mentionednot mentioned
kilogram-force kgflistedlistednot mentionednot mentionednot mentioned
calorie callistedlistedlistednot mentionednot mentioned
micron μlistedlistedlistednot mentionednot mentioned
x-unit xulistedlistednot mentionednot mentionednot mentioned
stilb sblistedlistednot mentionednot mentionednot mentioned
gamma (Mass) γlistedlistedlistednot mentionednot mentioned
γ (magnetic flux density) γlistedlistednot mentionednot mentionednot mentioned
lambda λlistedlistednot mentionednot mentionednot mentioned
jansky Jynot mentionednot mentionedlistednot mentionednot mentioned
millimetre of mercury mmHgnot mentionednot mentionednot mentionedlistednot mentioned

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

See also

Notes and references


  1. A footnote in the 9th SI Brochure gives an exact definition of the dalton.
  2. This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
  3. 1 2 3 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.
  4. The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.

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  1. 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.
  2. "2018 CODATA Value: atomic mass constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
  3. 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.