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] 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[edit]

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 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[edit]

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 unit au[note 2] accepted accepted
unified atomic mass unit u accepted footnote only
natural unit (n.u.) of speed c0 listed not mentioned
n.u. of action ħ listed not mentioned
n.u. of mass me listed not mentioned
n.u. of time ħ/(mec02) listed not mentioned
atomic unit (a.u.) of charge e listed not mentioned
a.u. of mass me listed not mentioned
a.u. of action ħ listed not mentioned
a.u. of length a0 listed not mentioned
a.u. of energy Eh listed not mentioned
a.u. of time ħ/Eh listed not mentioned
bar bar listed not mentioned
millimetre of mercury mmHg listed not mentioned
ångström Å listed not mentioned
nautical mile M listed not mentioned
barn b listed not mentioned
knot kn listed not mentioned
erg erg listed not mentioned
dyne dyn listed not mentioned
poise P listed not mentioned
stokes st listed not mentioned
stilb sb listed not mentioned
phot ph listed not mentioned
gal Gal listed footnote only
maxwell Mx listed not mentioned
gauss G listed not mentioned
œrsted Oe listed not 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[edit]

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[edit]

Notes and references[edit]


  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.


  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. ^ a b "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.