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While not an SI-unit, the litre may be used with SI units. It is equivalent to (10 cm)³ = (1 dm)³ = 10⁻³ m³.

While the International System of Units (SI) is used throughout the world in all fields, many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The authority behind the SI system, the General Conference on Weights and Measures, recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI.^[1]

Some units of time, angle, and legacy non-SI units have a long history of use. Most societies have used the solar day and its non-decimal subdivisions as a basis of time and, unlike the foot or the pound, these were the same regardless of where they were being measured. The radian, being ⁠1/2 $π$ ⁠ of a revolution, has mathematical advantages but is rarely used for navigation. Further, the units used in navigation around the world are similar. The tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are given below.^[2]

Most of these, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min = 60 s), the hour (3600 s), and the day (86400 s); the degree (for measuring plane angles, 1° = π⁄180 rad); and the electronvolt (a unit of energy, 1 eV = 1.602176634×10⁻¹⁹ J).^[2]

Units officially accepted for use with 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,^[2] listed as being accepted for use alongside SI-units, or for explanatory purposes.

Name	Symbol	Quantity	Value in SI units
minute	min	time	1 min = 60 s
hour	h		1 h = 60 min = 3600 s
day	d		1 d = 24 h = 1440 min = 86400 s
astronomical unit	au	length	1 au = 149597870700 m
degree	°	plane angle and phase angle	1° = (π⁄180) rad
arcminute	′		1′ = (1⁄60)° = (π⁄10800) rad
arcsecond	″		1″ = (1⁄60)′ = (1⁄3600)° = (π⁄648000) rad
hectare	ha	area	1 ha = 1 hm² = 10000 m²
litre	l, L	volume	1 L = 1 dm³ = 1000 cm³ = 0.001 m³
tonne	t	mass	1 t = 1 Mg = 1000 kg
dalton	Da	mass	1 Da = 1.66053906892(52)×10⁻²⁷ kg‍^[3]^{[note 1]}
electronvolt	eV	energy	1 eV = 1.602176634×10⁻¹⁹ J‍^[4]
neper	Np	logarithmic ratio quantity	1 Np = 1
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 sidenotes 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/s² = 0.01 m/s²
unified atomic mass unit^{[note 3]}	u	mass	1 u = 1 Da
volt-ampere reactive	var	reactive power	1 var = 1 VA

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.^[5] 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	m_e	omitted	omitted	omitted	listed	omitted
n.u. of time	ħ⁄(m_ec₀²)	omitted	omitted	omitted	listed	omitted
atomic unit (a.u.) of charge	e	omitted	omitted	omitted	listed	omitted
a.u. of mass	m_e	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	E_h	omitted	omitted	omitted	listed	omitted
a.u. of time	ħ⁄E_h	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.

Notes and references

Notes

^ A footnote in the 9th SI Brochure gives an exact definition of the dalton.
^ This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
^ 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.
^ The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.