Non-SI units mentioned in the SI

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] because the General Conference on Weights and Measures (CGPM) accepts their use as being multiples or submultiples of SI-units, they have important contemporary application worldwide, or they are otherwise commonly encountered worldwide.

Units officially accepted for use with the SI
Name Symbol Quantity Value in SI units
minute min time 1 min = 60 s
hour h time 1 h = 60 min = 3 600 s
day d time 1 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
minute ′ plane angle 1′ = (1/60)° = (π/10 800) rad
second ″ plane angle 1″ = (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 Da mass 1 Da = 1.66053906660(50)×10−27 kg[2] = 1.660 539 066 60(50) yg[note 1]
electronvolt eV energy 1 eV = 1.602176634×10−19 J = 160.217 663 4 zJ
neper Np logarithmic ratio quantity —
bel, decibel B, dB logarithmic ratio quantity —

Changes to units mentioned in the SI

With the publication of the 9th SI brochure in 2019, 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.

Name Symbol 1st-3rd SI Brochures 4-6th SI Brochures 7th SI Brochure 8th SI Brochure 9th SI Brochure
astronomical unitau[note 2] accepted not mentioned acceptedacceptedaccepted
Parsec pc accepted not mentioned not mentioned not mentioned not mentioned
neper Np not mentioned not mentioned accepted listed accepted
bel b not mentioned not mentioned accepted listed accepted
decibel db not mentioned not mentioned not mentioned listed accepted
unified atomic mass unitu accepted accepted acceptedacceptedfootnote only
dalton Da not mentioned not mentioned not mentioned accepted accepted
natural unit (n.u.) of speedc0 not mentioned not mentioned not mentionedlistednot mentioned
n.u. of actionħ not mentioned not mentioned not mentionedlistednot mentioned
n.u. of massme not mentioned not mentioned not mentionedlistednot mentioned
n.u. of timeħ/(mec02) not mentioned not mentioned not mentionedlistednot mentioned
atomic unit (a.u.) of chargee not mentioned not mentioned not mentionedlistednot mentioned
a.u. of massme not mentioned not mentioned not mentionedlistednot mentioned
a.u. of actionħ not mentioned not mentioned not mentionedlistednot mentioned
a.u. of lengtha0 not mentioned not mentioned not mentionedlistednot mentioned
a.u. of energyEh not mentioned not mentioned not mentionedlistednot mentioned
a.u. of timeħ/Eh not mentioned not mentioned not mentionedlistednot mentioned
nautical mileM temporarily accepted temporarily accepted temporarily acceptedlistednot mentioned
knotkn temporarily accepted temporarily accepted temporarily acceptedlistednot mentioned
ångströmÅ temporarily accepted temporarily accepted temporarily acceptedlistednot mentioned
are a temporarily accepted temporarily accepted temporarily accepted not mentioned not mentioned
hectare ha temporarily accepted temporarily accepted temporarily accepted accepted accepted
barnb temporarily accepted temporarily accepted temporarily acceptedlistednot mentioned
barbar temporarily accepted temporarily accepted temporarily acceptedlistednot mentioned
standard atmosphere atm temporarily accepted listed listed not mentioned not mentioned
galGal temporarily accepted temporarily accepted listedlistedfootnote only
curie C temporarily accepted temporarily accepted listed not mentioned not mentioned
roentgen R temporarily accepted temporarily accepted listed not mentioned not mentioned
rad rad temporarily accepted temporarily accepted listed not mentioned not mentioned
rem rem not mentioned temporarily accepted listed not mentioned not mentioned
ergerg listed listed listedlistednot mentioned
dynedyn listed listed listedlistednot mentioned
poiseP listed listed listedlistednot mentioned
stokesst listed listed listedlistednot mentioned
maxwellMx listed listed listedlistednot mentioned
gaussG listed listed listedlistednot mentioned
ørstedOe listed listed listedlistednot mentioned
stilbsb listed listed listedlistednot mentioned
photph listed listed listedlistednot mentioned
fermi listed listed listednot mentionednot mentioned
carat listed listed listednot mentionednot mentioned
torr listed listed listednot mentionednot mentioned
kilogram-forcekgf listed listed not mentionednot mentionednot mentioned
caloriecal listed listed listednot mentionednot mentioned
micronμ listed listed listednot mentionednot mentioned
x-unitxu listed listed not mentionednot mentionednot mentioned
stilbsb listed listed not mentionednot mentionednot mentioned
gamma (Mass)γ listed listed listednot mentionednot mentioned
γ (magnetic flux density)γ listed listed not mentionednot mentionednot mentioned
lambdaλ listed listed not mentionednot mentionednot mentioned
jansky Jy not mentioned not mentioned listed not mentioned not mentioned
millimetre of mercurymmHg not mentioned not mentioned not mentionedlistednot mentioned

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

  • "accepted": The unit is accepted for use with the SI.
  • "temporarily accepted": 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 only": The unit is defined in a footnote or side note, without any mention in the main text.
  • "not mentioned": There is no mention of the unit in the brochure, excluding historical appendices.


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.

Name Symbol Quantity Equivalent SI unit
gal[note 3] Gal acceleration 1 Gal = 1 cm⋅s−2 = 0.01 m⋅s−2
unified atomic mass unit[note 4] u mass 1 u = 1 Da = 1.66053906660(50)×10−27 kg[2]
volt-ampere reactive var reactive power 1 var = 1 V⋅A

See also

Notes and references

Notes
  1. The latest CODATA value has been substituted for the value listed in the 9th SI Brochure, since it supersedes that value. An associated footnote gives an exact definition of the dalton.
  2. The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.
  3. This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
  4. As of 2019, the SI Brochure no longer lists the unified atomic mass unit (symbol u) as being accepted for use with the SI units, though it notes its equivalence with the dalton in a footnote.

References
  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.
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