Other non-SI units

Some non-SI units, such as the nautical mile, have previously been accepted for use with the SI, but no longer have any special status.

Examples of non-SI units that are no longer accepted for use with the SI

Name	Symbol	Quantity	SI units
ångström, angstrom	Å	length	10^-10 m
$1 \mspace{4mu} \text{A} \mspace{6mu} = \dfrac{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770}{(10^{10})(299 \mspace{4mu} 792 \mspace{4mu} 458)} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{A} \mspace{6mu} \approx 3.066 \mspace{4mu} \mspace{4mu} 331 \mspace{4mu} 898 \mspace{4mu} 849 \mspace{4mu} 836 \mspace{4mu} 976 \times10^{-9} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$
nautical mile	nmi	length	1852 m
$1 \mspace{4mu} \text{nmi} \mspace{6mu} = \dfrac{(1 \mspace{4mu} 852)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{nmi} \mspace{6mu} \approx 5.678 \mspace{4mu} 846 \mspace{4mu} 676 \mspace{4mu} 669 \mspace{4mu} 898 \mspace{4mu} 080 \times10^{4} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$
parsec	pc	length	≈30.86 Pm
$1 \mspace{4mu} \text{pc} \mspace{6mu} = \dfrac{(149 \mspace{4mu} 597 \mspace{4mu} 870 \mspace{4mu} 700)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{(\text{tan} \mspace{4mu} 1^{\prime \prime})(299 \mspace{4mu} 792 \mspace{4mu} 458)} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{pc} \mspace{6mu} \approx 9.461 \mspace{4mu} 711 \mspace{4mu} 597 \mspace{4mu} 618 \mspace{4mu} 666 \mspace{4mu} 090 \times10^{17} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$
barn	b	area	10^-28 m²
$1 \mspace{4mu} \text{b} \mspace{6mu} = \dfrac{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^2}{(10^{28})(299 \mspace{4mu} 792 \mspace{4mu} 458)^2} \mspace{6mu} \dfrac{c^2}{{\Delta \nu _{Cs}}^2}\\ \\ \\ 1 \mspace{4mu} \text{b} \mspace{6mu} \approx 9.402 \mspace{4mu} 391 \mspace{4mu} 313 \mspace{4mu} 904 \mspace{4mu} 046 \mspace{4mu} 861 \times10^{-26} \mspace{6mu} c^2 \mspace{4mu} {\Delta \nu _{Cs}}^{-2}$
knot	kn	speed	¹⁸⁵²⁄₃₆₀₀ m s^-1
$1 \mspace{4mu} \text{kn} \mspace{6mu} = \dfrac{1 \mspace{4mu} 852}{(3 \mspace{4mu} 600)(299 \mspace{4mu} 792 \mspace{4mu} 458)} \mspace{6mu} c\\ \\ \\ 1 \mspace{4mu} \text{kn} \mspace{6mu} \approx 1.716 \mspace{4mu} 001 \mspace{4mu} 956 \mspace{4mu} 408 \mspace{4mu} 271 \mspace{4mu} 099 \mspace{4mu} 483 \times10^{-9} \mspace{6mu} c$
bar	bar	pressure	10⁵ Pa
$1 \mspace{4mu} \text{bar} \mspace{6mu} = \dfrac{(10^5)(299 \mspace{4mu} 792 \mspace{4mu} 458)^3}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^4} \mspace{6mu} \dfrac{h \mspace{4mu} {\Delta \nu _{Cs}}^4}{c^3}\\ \\ \\ 1 \mspace{4mu} \text{bar} \mspace{6mu} \approx 5.694 \mspace{4mu} 382 \mspace{4mu} 339 \mspace{4mu} 804 \mspace{4mu} 557 \mspace{4mu} 242 \times10^{23} \mspace{6mu} h \mspace{4mu} c^{-3} \mspace{4mu} {\Delta \nu _{Cs}}^4$

Some non-SI units, that have never been accepted for use with the SI, continue to be used in certain fields. Some of these units, such as the dioptre, are highly compatible with the SI. Others, such as the enzyme unit, are better replaced by a coherent SI unit with an appropriate prefix. A few examples of such non-SI units, and their definitions, are listed below.

Examples of non-SI units used in certain fields

Name	Symbol	Quantity		SI units
light year	ly	length	9 460 730 472 580 800 m
$1 \mspace{4mu} \text{ly} \mspace{6mu} = (31 \mspace{4mu} 557 \mspace{4mu} 600)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770) \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{ly} \mspace{6mu} = 2.900 \mspace{4mu} 973 \mspace{4mu} 963 \mspace{4mu} 449 \mspace{4mu} 52 \times10^{17} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$
dioptre	D	optical refractive power	m^-1
$1 \mspace{4mu} \text{D} \mspace{6mu} = \dfrac{299 \mspace{4mu} 792 \mspace{4mu} 458}{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770} \mspace{6mu} \dfrac{\Delta \nu _{Cs}}{c}\\ \\ \\ 1 \mspace{4mu} \text{D} \mspace{6mu} \approx 3.261 \mspace{4mu} 225 \mspace{4mu} 571 \mspace{4mu} 749 \mspace{4mu} 405 \mspace{4mu} 557 \times10^{-2} \mspace{6mu} c^{-1} \mspace{4mu} \Delta \nu _{Cs}$
millimetre of mercury	mmHg	pressure	≈133.322 Pa
$1 \mspace{4mu} \text{mmHg} \mspace{6mu} \approx \dfrac{(133.322)(299 \mspace{4mu} 792 \mspace{4mu} 458)^3}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^4} \mspace{6mu} \dfrac{h \mspace{4mu} {\Delta \nu _{Cs}}^4}{c^3}\\ \\ \\ 1 \mspace{4mu} \text{mmHg} \mspace{6mu} \approx 7.591 \mspace{4mu} 864 \mspace{4mu} 423 \mspace{4mu} 074 \mspace{4mu} 231 \mspace{4mu} 806 \times10^{20} \mspace{6mu} h \mspace{4mu} c^{-3} \mspace{4mu} {\Delta \nu _{Cs}}^4$
R-value	R	thermal insulance	kg^-1 s³ K
$1 \mspace{4mu} \text{R} \mspace{6mu} = \dfrac{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^3(1.380 \mspace{4mu} 649 \times 10^{-23})}{(299 \mspace{4mu} 792 \mspace{4mu} 458)^2} \mspace{6mu} \dfrac{c^2}{{\Delta \nu _{Cs}}^3 \mspace{4mu} k}\\ \\ \\ 1 \mspace{4mu} \text{R} \mspace{6mu} \approx 1.193 \mspace{4mu} 332 \mspace{4mu} 499 \mspace{4mu} 625 \mspace{4mu} 075 \mspace{4mu} 118 \times10^{-10} \mspace{6mu} c^2 \mspace{4mu} {\Delta \nu _{Cs}}^{-3} \mspace{4mu} k^{-1}$
tog	tog	thermal insulance	10^-1 kg^-1 s³ K
$1 \mspace{4mu} \text{tog} \mspace{6mu} = \dfrac{(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^3(1.380 \mspace{4mu} 649 \times 10^{-23})}{(10)(299 \mspace{4mu} 792 \mspace{4mu} 458)^2} \mspace{6mu} \dfrac{c^2}{{\Delta \nu _{Cs}}^3 \mspace{4mu} k}\\ \\ \\ 1 \mspace{4mu} \text{tog} \mspace{6mu} \approx 1.193 \mspace{4mu} 332 \mspace{4mu} 499 \mspace{4mu} 625 \mspace{4mu} 075 \mspace{4mu} 118 \times10^{-11} \mspace{6mu} c^2 \mspace{4mu} {\Delta \nu _{Cs}}^{-3} \mspace{4mu} k^{-1}$
enzyme unit	U	catalytic activity	(¹⁄₆₀) × 10^-6 s^-1 mol
$1 \mspace{4mu} \text{U} \mspace{6mu} = \dfrac{6.022 \mspace{4mu} 140 \mspace{4mu} 76 \times 10^{23}}{(6 \times 10^7)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)} \mspace{6mu} \dfrac{\Delta \nu _{Cs}}{N_A}\\ \\ \\ 1 \mspace{4mu} \text{U} \mspace{6mu} \approx 1.091 \mspace{4mu} 841 \mspace{4mu} 979 \mspace{4mu} 292 \mspace{4mu} 799 \mspace{4mu} 049 \times10^{6} \mspace{6mu} \Delta \nu _{Cs} \mspace{4mu} {N_A}^{-1}$
gradian, gon	gon	plane angle	^π⁄₂₀₀ rad
$1 \mspace{4mu} \text{gon} = \dfrac{\pi}{200} \mspace{6mu} \text{rad}$

Other non-SI units not recommended for use

There are many more non-SI units, which are too numerous to list here, which are either of historical interest, or are still used but only in specialised fields (for example, the barrel of oil) or in particular countries (the inch, foot, yard and mile). There is no case for continuing to use these units in modern scientific and technical work.