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 = 1,440 min = 86,400 s
astronomical unit au length 1 au = 149597870700 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 volume 1 l = 1 dm3 = 1,000 cm3 = 0.001 m3
tonne t mass 1 t = 103 kg
dalton Da mass 1 Da = 1.66053906660(50)×10−27 kg[2] = 1.660 539 066 60(50) ym[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 8th SI brochure.[3] The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.

Name Symbol 8th SI Brochure 9th SI Brochure
astronomical unitau[note 2]acceptedaccepted
unified atomic mass unituacceptedfootnote only
natural unit (n.u.) of speedc0listednot mentioned
n.u. of actionħlistednot mentioned
n.u. of massmelistednot mentioned
n.u. of timeħ/(mec02)listednot mentioned
atomic unit (a.u.) of chargeelistednot mentioned
a.u. of massmelistednot mentioned
a.u. of actionħlistednot mentioned
a.u. of lengtha0listednot mentioned
a.u. of energyEhlistednot mentioned
a.u. of timeħ/Ehlistednot mentioned
barbarlistednot mentioned
millimetre of mercurymmHglistednot mentioned
ångströmÅlistednot mentioned
nautical mileMlistednot mentioned
barnblistednot mentioned
knotknlistednot mentioned
ergerglistednot mentioned
dynedynlistednot mentioned
poisePlistednot mentioned
stokesstlistednot mentioned
stilbsblistednot mentioned
photphlistednot mentioned
galGallistedfootnote only
maxwellMxlistednot mentioned
gaussGlistednot mentioned
œrstedOelistednot mentioned

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

  • "accepted": The unit is accepted for use with the SI.
  • "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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