SI Units

The International System of Units (SI, or Système International d'Unités), is a set of measurement standards which defines (almost) all standards in terms of uniform natural phenomena, and form the base of the metric system.

Base SI Units

There are seven base SI units, with many derived units made from combinations of these. Base SI units are mutually independent. They consist of:

Quantity	Unit	Abbreviation	Definition
Time	Second	s	Duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of an atom of Cs-133
Length	Metre	m	Distance that light travels in a vacuum in 1/299,792,458^th of a second
Current	Ampere	A	The constant current that would produce a force of 2x10^-7 Newton between two conductors of infinite length and negligible cross section in a vacuum
Temperature	Kelvin	°K	1/273.16^th of the triple point of water. The triple point is the temperature at which a substance exists in equilibrium in all three phases (solid, liquid, gas).
Amount	Mole	mol	The amount of substance which contains as many elementary entities as in 0.012kg of Carbon 12
Luminous Intensity	Candella	cd	Luminous intensity of a source which emits monochromatic radiation at 540 x 10¹² Hz at radiant intensity of 1/683 watts per steradian
Mass	Kilogram	kg	Weight of the International Prototype Kilogram (IPK)

Derived Units

Quantity	Unit	Abbreviation	Conversion to Base SI Units	Definition
Area	Square metre	m²	$m^2$
Velocity	Metre per Second	m.s^-1	$m \over s$
Acceleration	Metre per Second per Second	m.s^-2	$m \over s^2$
Force	Newton	N	$N={kg.m \over s^2}$	Force required to accelerate 1kg at 1m.s^-2
Pressure	Pascal	Pa	$Pa = {N \over m^2} = {kg \over m.s^2}$	Force per area
Energy/Work/Quantity of Heat	Joule	J	$J = N.m = {kg.m^2 \over s^2 }$	Energy converted when 1N is applied to 1kg over 1m
Dose Equivalence	Sievert	Sv	$Sv = {J \over kg} = {m^2 \over s^2 }$	Radiation dose per mass
Power	Watt	W	$W = {J \over s} = {kg.m^2 \over s^3 }$	Rate of energy conversion per second
Electromotive Force	Volt	V	$V = {W \over A} = {kg.m^2 \over A.s^3 }$	Measure of electrical potential energy

References

Physical Measurement Laboratory. National Institute of Standards and Technology.
- And various subpages

Last updated 2017-09-22

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