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 hm^2 = 10 000 m^2
litre	l, L	volume	1 L = 1 dm^3 = 1 000 cm^3 = 0.001 m^3
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 official 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 Da
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	omitted	accepted	accepted	accepted
parsec	pc	accepted	omitted	omitted	omitted	omitted
neper	Np	omitted	omitted	accepted	listed	accepted
bel	b	omitted	omitted	accepted	listed	accepted
decibel	db	omitted	omitted	omitted	listed	accepted
unified atomic mass unit	u	accepted	accepted	accepted	accepted	footnote
dalton	Da	omitted	omitted	footnote	accepted	accepted
natural unit (n.u.) of speed	c0	omitted	omitted	omitted	listed	omitted
n.u. of action	ħ	omitted	omitted	omitted	listed	omitted
n.u. of mass	me	omitted	omitted	omitted	listed	omitted
n.u. of time	ħ/(mec0^2)	omitted	omitted	omitted	listed	omitted
atomic unit (a.u.) of charge	e	omitted	omitted	omitted	listed	omitted
a.u. of mass	me	omitted	omitted	omitted	listed	omitted
a.u. of action	ħ	omitted	omitted	omitted	listed	omitted
a.u. of length	a0	omitted	omitted	omitted	listed	omitted
a.u. of energy	Eh	omitted	omitted	omitted	listed	omitted
a.u. of time	ħ/Eh	omitted	omitted	omitted	listed	omitted
nautical mile	M	temporary	temporary	temporary	listed	omitted
knot	kn	temporary	temporary	temporary	listed	omitted
ångström	Å	temporary	temporary	temporary	listed	omitted
are	a	temporary	temporary	temporary	omitted	omitted
hectare	ha	temporary	temporary	temporary	accepted	accepted
barn	b	temporary	temporary	temporary	listed	omitted
bar	bar	temporary	temporary	temporary	listed	omitted
standard atmosphere	atm	temporary	listed	listed	omitted	omitted
gal	Gal	temporary	temporary	listed	listed	footnote
curie	Ci	temporary	temporary	listed	omitted	omitted
roentgen	R	temporary	temporary	listed	omitted	omitted
rad	rad	temporary	temporary	listed	omitted	omitted
rem	rem	omitted	temporary	listed	omitted	omitted
erg	erg	listed	listed	listed	listed	omitted
dyne	dyn	listed	listed	listed	listed	omitted
poise	P	listed	listed	listed	listed	omitted
stokes	st	listed	listed	listed	listed	omitted
maxwell	Mx	listed	listed	listed	listed	omitted
gauss	G	listed	listed	listed	listed	omitted
ørsted	Oe	listed	listed	listed	listed	omitted
phot	ph	listed	listed	listed	listed	omitted
fermi		listed	listed	listed	omitted	omitted
metric carat		listed	listed	listed	omitted	omitted
torr	Torr	listed	listed	listed	omitted	omitted
kilogram-force	kgf	listed	listed	omitted	omitted	omitted
calorie	cal	listed	listed	listed	omitted	omitted
micron	μ	listed	listed	listed	omitted	omitted
x-unit	xu	listed	listed	omitted	omitted	omitted
stilb	sb	listed	listed	omitted	omitted	omitted
gamma (Mass)	γ	listed	listed	listed	omitted	omitted
γ (magnetic flux density)	γ	listed	listed	omitted	omitted	omitted
lambda	λ	listed	listed	omitted	omitted	omitted
jansky	Jy	omitted	omitted	listed	omitted	omitted
millimetre of mercury	mmHg	omitted	omitted	omitted	listed	omitted

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

• "accepted": The unit is accepted for use with the SI.
• "temporary": 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": The unit is defined in a footnote or side note, without any mention in the main text.
• "omitted": There is no mention of the unit in the brochure, excluding historical appendices.

See also

• International System of Quantities
• International vocabulary of metrology
• Metric prefix
• SI base unit
• SI derived unit

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.