Non-SI units accepted for use with SI

SI coherent units have the advantage that unit conversions are not required when inserting particular values for quantities into quantity equations. Nonetheless it is recognised that some non-SI units are widely used and are expected to continue to be used for many years. Therefore, some non-SI units are accepted for use with the SI. If these units are used, some advantages of the SI are lost.

The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time.

Currently, there are fifteen non-SI units accepted for use with the SI.

Units in everyday use

Name	Symbol	Quantity	SI units
tonne	t	mass	10³ kg
$1 \mspace{4mu} \text{t} \mspace{6mu} = \dfrac{(10^3) (299 \mspace{4mu} 792 \mspace{4mu} 458)^2}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)} \mspace{6mu} \dfrac{h \mspace{4mu} \Delta \nu _{Cs}}{c^2}\\ \\ \\ 1 \mspace{4mu} \text{t} \mspace{6mu} \approx 1.475 \mspace{4mu} 521 \mspace{4mu} 399 \mspace{4mu} 735 \mspace{4mu} 270 \mspace{4mu} 916 \times10^{43} \mspace{6mu} h \mspace{4mu} c^{-2} \mspace{4mu} \Delta \nu _{Cs}$
hectare	ha	area	10⁴ m²
$1 \mspace{4mu} \text{ha} \mspace{6mu} = \dfrac{(10^4) (9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^2}{(299 \mspace{4mu} 792 \mspace{4mu} 458)^2} \mspace{6mu} \dfrac{c^2}{{\Delta \nu _{Cs}}^2}\\ \\ \\ 1 \mspace{4mu} \text{ha} \mspace{6mu} \approx 9.402 \mspace{4mu} 391 \mspace{4mu} 313 \mspace{4mu} 904 \mspace{4mu} 046 \mspace{4mu} 861 \times10^{6} \mspace{6mu} c^2 \mspace{4mu} {\Delta \nu _{Cs}}^{-2}$
litre	L, l	volume	10^-3 m³
$1 \mspace{4mu} \text{L} \mspace{6mu} = \dfrac{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^3}{(10^3) (299 \mspace{4mu} 792 \mspace{4mu} 458)^3} \mspace{6mu} \dfrac{c^3}{{\Delta \nu _{Cs}}^3}\\ \\ \\ 1 \mspace{4mu} \text{L} \mspace{6mu} \approx 28.830 \mspace{4mu} 852 \mspace{4mu} 411 \mspace{4mu} 292 \mspace{4mu} 609 \mspace{4mu} 605 \mspace{4mu} 475 \mspace{6mu} c^3 \mspace{4mu} {\Delta \nu _{Cs}}^{-3}$
minute	min	time	60 s
$1 \mspace{4mu} \text{min} = (60)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770) \mspace{6mu} \dfrac{1}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{min} = 551 \mspace{4mu} 557 \mspace{4mu} 906 \mspace{4mu} 200 \mspace{6mu} {\Delta \nu _{Cs}}^{-1}$
hour	h	time	3600 s
$1 \mspace{4mu} \text{h} = (3 \mspace{4mu} 600)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770) \mspace{6mu} \dfrac{1}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{h} = 33 \mspace{4mu} 093 \mspace{4mu} 474 \mspace{4mu} 372 \mspace{4mu} 000 \mspace{6mu} {\Delta \nu _{Cs}}^{-1}$
day	d	time	86 400 s
$1 \mspace{4mu} \text{d} = (86 \mspace{4mu} 400)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770) \mspace{6mu} \dfrac{1}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{d} = 794 \mspace{4mu} 243 \mspace{4mu} 384 \mspace{4mu} 928 \mspace{4mu} 000 \mspace{6mu} {\Delta \nu _{Cs}}^{-1}$
degree	°	plane angle, phase angle	^π⁄₁₈₀ rad
$1^\circ = \dfrac{\pi}{180} \mspace{6mu} \text{rad}$
minute	′	plane angle, phase angle	^π⁄_10 800 rad
$1^\prime = \dfrac{\pi}{10 \ 800} \mspace{6mu} \text{rad}$
second	″	plane angle, phase angle	^π⁄_648 000 rad
$1^{\prime \prime} = \dfrac{\pi}{648 \ 000} \mspace{6mu} \text{rad}$

Units used in certain fields of science

Name	Symbol	Quantity		SI units
dalton	Da	mass	1.660 539 068 92(52) × 10⁻²⁷ kg
$1 \mspace{4mu} \text{Da} \mspace{6mu} = \dfrac{(1.660 \mspace{4mu} 539 \mspace{4mu} 068 \mspace{4mu} 92(52) \times10^{-27}) (299 \mspace{4mu} 792 \mspace{4mu} 458)^2}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)} \mspace{6mu} \dfrac{h \mspace{4mu} \Delta \nu _{Cs}}{c^2}\\ \\ \\ 1 \mspace{4mu} \text{Da} \mspace{6mu} = 2.450 \mspace{4mu} 160 \mspace{4mu} 931 \mspace{4mu} 29(77) \times10^{13} \mspace{6mu} h \mspace{4mu} c^{-2} \mspace{4mu} \Delta \nu _{Cs}$
electronvolt	eV	energy	1.602 176 634 × 10⁻¹⁹ J
$1 \mspace{4mu} \text{eV} \mspace{6mu} = \dfrac{1.602 \mspace{4mu} 176 \mspace{4mu} 634 \times 10^{-19}}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)} \mspace{6mu} h \mspace{4mu} \Delta \nu _{Cs}\\ \\ \\ 1 \mspace{4mu} \text{eV} \mspace{6mu} \approx 2.630 \mspace{4mu} 355 \mspace{4mu} 813 \mspace{4mu} 855 \mspace{4mu} 163 \mspace{4mu} 441 \times10^{4} \mspace{6mu} h \mspace{4mu} \Delta \nu _{Cs}$
astronomical unit	au	length	149 597 870 700 m
$1 \mspace{4mu} \text{au} \mspace{6mu} = \dfrac{(149 \mspace{4mu} 597 \mspace{4mu} 870 \mspace{4mu} 700)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{au} \mspace{6mu} \approx 4.587 \mspace{4mu} 167 \mspace{4mu} 229 \mspace{4mu} 274 \mspace{4mu} 233 \mspace{4mu} 907 \times10^{12} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$

Logarithmic units

Name	Symbol	Quantity	SI units
neper	Np	logarithmic ratio

bel	B	logarithmic ratio

decibel	dB	logarithmic ratio

Any non-SI unit that is not included in the above tables is, by definition, not accepted for use with the SI.