# 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 unit au[note 4] accepted not mentioned accepted accepted accepted
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 unit[note 3] u accepted accepted accepted accepted footnote only
dalton[note 3] Da not mentioned not mentioned footnote only accepted accepted
natural unit (n.u.) of speed c0 not mentioned not mentioned not mentioned listed not mentioned
n.u. of action ħ not mentioned not mentioned not mentioned listed not mentioned
n.u. of mass me not mentioned not mentioned not mentioned listed not mentioned
n.u. of time ħ/(mec02) not mentioned not mentioned not mentioned listed not mentioned
atomic unit (a.u.) of charge e not mentioned not mentioned not mentioned listed not mentioned
a.u. of mass me not mentioned not mentioned not mentioned listed not mentioned
a.u. of action ħ not mentioned not mentioned not mentioned listed not mentioned
a.u. of length a0 not mentioned not mentioned not mentioned listed not mentioned
a.u. of energy Eh not mentioned not mentioned not mentioned listed not mentioned
a.u. of time ħ/Eh not mentioned not mentioned not mentioned listed not mentioned
nautical mile M temporarily accepted temporarily accepted temporarily accepted listed not mentioned
knot kn temporarily accepted temporarily accepted temporarily accepted listed not mentioned
ångström Å temporarily accepted temporarily accepted temporarily accepted listed not mentioned
are a temporarily accepted temporarily accepted temporarily accepted not mentioned not mentioned
hectare ha temporarily accepted temporarily accepted temporarily accepted accepted accepted
barn b temporarily accepted temporarily accepted temporarily accepted listed not mentioned
bar bar temporarily accepted temporarily accepted temporarily accepted listed not mentioned
standard atmosphere atm temporarily accepted listed listed not mentioned not mentioned
gal Gal temporarily accepted temporarily accepted listed listed footnote only
curie Ci 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
erg erg listed listed listed listed not mentioned
dyne dyn listed listed listed listed not mentioned
poise P listed listed listed listed not mentioned
stokes st listed listed listed listed not mentioned
maxwell Mx listed listed listed listed not mentioned
gauss G listed listed listed listed not mentioned
ørsted Oe listed listed listed listed not mentioned
phot ph listed listed listed listed not mentioned
fermi listed listed listed not mentioned not mentioned
metric carat listed listed listed not mentioned not mentioned
torr Torr listed listed listed not mentioned not mentioned
kilogram-force kgf listed listed not mentioned not mentioned not mentioned
calorie cal listed listed listed not mentioned not mentioned
micron μ listed listed listed not mentioned not mentioned
x-unit xu listed listed not mentioned not mentioned not mentioned
stilb sb listed listed not mentioned not mentioned not mentioned
gamma (Mass) γ listed listed listed not mentioned not mentioned
γ (magnetic flux density) γ listed listed not mentioned not mentioned not mentioned
lambda λ listed listed not mentioned not mentioned not mentioned
jansky Jy not mentioned not mentioned listed not mentioned not mentioned
millimetre of mercury mmHg not mentioned not mentioned not mentioned listed not 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.

## 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. ^ a b c 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. ^ 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.