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

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 and phase angle 1° = (π/180) rad
arcminute ′ plane angle and phase angle 1′ = (1/60)° = (π/10 800) rad
arcsecond ″ plane angle and phase 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][note 1]
electronvolt eV energy 1 eV = 1.602176634×10−19 J
neper Np logarithmic ratio quantity —
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.

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 2] Gal acceleration 1 Gal = 1 cm⋅s−2 = 0.01 m⋅s−2
unified atomic mass unit[note 3] u mass 1 u =1.66053906660(50)×10−27 kg[2] 
volt-ampere reactive var reactive power 1 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.

Name Symbol 1st–3rd SI Brochures 4–6th SI Brochures 7th SI Brochure 8th SI Brochure 9th SI Brochure
astronomical unitau[note 4] acceptednot mentionedacceptedacceptedaccepted
parsecpc acceptednot mentionednot mentionednot mentionednot mentioned
neperNp not mentionednot mentionedacceptedlistedaccepted
belb not mentionednot mentionedacceptedlistedaccepted
decibeldb not mentionednot mentionednot mentionedlistedaccepted
unified atomic mass unit[note 3]u acceptedacceptedacceptedacceptedfootnote only
dalton[note 3]Da not mentionednot mentionedfootnote onlyacceptedaccepted
natural unit (n.u.) of speedc0 not mentionednot mentionednot mentionedlistednot mentioned
n.u. of actionħ not mentionednot mentionednot mentionedlistednot mentioned
n.u. of massme not mentionednot mentionednot mentionedlistednot mentioned
n.u. of timeħ/(mec02) not mentionednot mentionednot mentionedlistednot mentioned
atomic unit (a.u.) of chargee not mentionednot mentionednot mentionedlistednot mentioned
a.u. of massme not mentionednot mentionednot mentionedlistednot mentioned
a.u. of actionħ not mentionednot mentionednot mentionedlistednot mentioned
a.u. of lengtha0 not mentionednot mentionednot mentionedlistednot mentioned
a.u. of energyEh not mentionednot mentionednot mentionedlistednot mentioned
a.u. of timeħ/Eh not mentionednot mentionednot mentionedlistednot mentioned
nautical mileM temporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
knotkn temporarily acceptedtemporarily acceptedtemporarily acceptedlistednot mentioned
ångströmÅ temporarily accepted temporarily accepted temporarily accepted listed not mentioned
area temporarily accepted temporarily accepted temporarily accepted not mentioned not mentioned
hectareha temporarily accepted temporarily accepted temporarily accepted accepted accepted
barnb temporarily accepted temporarily accepted temporarily accepted listed not mentioned
barbar temporarily accepted temporarily accepted temporarily accepted listed not mentioned
standard atmosphereatm temporarily accepted listed listed not mentioned not mentioned
galGal temporarily accepted temporarily accepted listed listed footnote only
curieCi temporarily accepted temporarily accepted listed not mentioned not mentioned
roentgenR temporarily accepted temporarily accepted listed not mentioned not mentioned
radrad temporarily accepted temporarily accepted listed not mentioned not mentioned
remrem not mentioned temporarily accepted listed not mentioned not mentioned
ergerg listedlistedlistedlistednot mentioned
dynedyn listedlistedlistedlistednot mentioned
poiseP listedlistedlistedlistednot mentioned
stokesst listedlistedlistedlistednot mentioned
maxwellMx listedlistedlistedlistednot mentioned
gaussG listedlistedlistedlistednot mentioned
ørstedOe listedlistedlistedlistednot mentioned
photph listedlistedlistedlistednot mentioned
fermi listedlistedlistednot mentionednot mentioned
metric carat listedlistedlistednot mentionednot mentioned
torrTorr listedlistedlistednot mentionednot mentioned
kilogram-forcekgf listedlistednot mentionednot mentionednot mentioned
caloriecal listedlistedlistednot mentionednot mentioned
micronμ listedlistedlistednot mentionednot mentioned
x-unitxu listedlistednot mentionednot mentionednot mentioned
stilbsb listedlistednot mentionednot mentionednot mentioned
gamma (Mass)γ listedlistedlistednot mentionednot mentioned
γ (magnetic flux density)γ listedlistednot mentionednot mentionednot mentioned
lambdaλ listedlistednot mentionednot mentionednot mentioned
janskyJy not mentionednot mentionedlistednot mentionednot mentioned
millimetre of mercurymmHg not mentionednot mentionednot 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.

See also

Notes and references

Notes
  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. 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.

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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