Non-SI units mentioned in the SI explained

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.

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 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.

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, since), the hour, and the day ; the degree (for measuring plane angles, and the electronvolt (a unit of energy,).

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

Symbol	Quantity	Value in SI units
min		=
h		= =
d		= = =
au		=
°		=
′		= =
″		= = =
ha		= =
l, L		= = =
t		= =
Da		=
eV		=
Np	logarithmic ratio quantity	1 Np = 1
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.

Symbol	Quantity	Equivalent official unit
Gal		= =
u	mass	=
var	reactive power	=

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.^[2] 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
	au	accepted	omitted	accepted	accepted	accepted
	pc	accepted	omitted	omitted	omitted	omitted
	Np	omitted	omitted	accepted	listed	accepted
	B	omitted	omitted	accepted	listed	accepted
	dB	omitted	omitted	omitted	listed	accepted
	u	accepted	accepted	accepted	accepted	footnote
	Da	omitted	omitted	footnote	accepted	accepted
natural unit (n.u.) of speed	c	omitted	omitted	omitted	listed	omitted
n.u. of action	ħ	omitted	omitted	omitted	listed	omitted
n.u. of mass	m	omitted	omitted	omitted	listed	omitted
n.u. of time		omitted	omitted	omitted	listed	omitted
atomic unit (a.u.) of charge	e	omitted	omitted	omitted	listed	omitted
a.u. of mass	m	omitted	omitted	omitted	listed	omitted
a.u. of action	ħ	omitted	omitted	omitted	listed	omitted
a.u. of length	a	omitted	omitted	omitted	listed	omitted
a.u. of energy	E	omitted	omitted	omitted	listed	omitted
a.u. of time		omitted	omitted	omitted	listed	omitted
	M	temporary	temporary	temporary	listed	omitted
	kn	temporary	temporary	temporary	listed	omitted
	Å	temporary	temporary	temporary	listed	omitted
	a	temporary	temporary	temporary	omitted	omitted
	ha	temporary	temporary	temporary	accepted	accepted
	b	temporary	temporary	temporary	listed	omitted
	bar	temporary	temporary	temporary	listed	omitted
	atm	temporary	listed	listed	omitted	omitted
	Gal	temporary	temporary	listed	listed	footnote
	Ci	temporary	temporary	listed	omitted	omitted
	R	temporary	temporary	listed	omitted	omitted
	rad	temporary	temporary	listed	omitted	omitted
	rem	omitted	temporary	listed	omitted	omitted
	erg	listed	listed	listed	listed	omitted
	dyn	listed	listed	listed	listed	omitted
	P	listed	listed	listed	listed	omitted
	st	listed	listed	listed	listed	omitted
	Mx	listed	listed	listed	listed	omitted
	G	listed	listed	listed	listed	omitted
	Oe	listed	listed	listed	listed	omitted
phot	ph	listed	listed	listed	listed	omitted
		listed	listed	listed	omitted	omitted
		listed	listed	listed	omitted	omitted
	Torr	listed	listed	listed	omitted	omitted
	kgf	listed	listed	omitted	omitted	omitted
	cal	listed	listed	listed	omitted	omitted
	μ	listed	listed	listed	omitted	omitted
	xu	listed	listed	omitted	omitted	omitted
	sb	listed	listed	omitted	omitted	omitted
	γ	listed	listed	listed	omitted	omitted
	γ	listed	listed	omitted	omitted	omitted
	λ	listed	listed	omitted	omitted	omitted
	Jy	omitted	omitted	listed	omitted	omitted
	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

References

Notes and References

[Bureau international des poids et mesures]
[Bureau international des poids et mesures]