Base units

There are 7 mutally independent base units, from which all other physical units are derived. They are based on the definitions of some universal constants of nature, such as, among others, the speed of light in vacuum, the constants of Avogadro, Boltzmann and Planck, and the electron charge.

unit	symbol	quantity	definition
second	s	time	ground-state hyperfine transition frequency ν_hf of ¹³³Cs = 9,192,631,770 Hz ^*)
meter	m	length	speed of light c in vacuum = 299,792,458 m·s^-1
kilogram	kg	mass	Planck's constant h = 6.626,070,15×10^-34 J·s ^**)
ampere	A	current	electron charge e = 1.602,176,634×10^-19 A·s
kelvin	K	temperature	Boltzmann's constant k_B = 1.380,649×10^-23 J·K^-1 ^**)
mole	mol	matter	Avogadro's constant N_A = 6.022,140,76×10²³ mole^-1
candela	cd	luminous intensity	1 candela = luminous intensity of standard source ^***)

^*) 1 Hz = 1 s^-1
^**) 1 J = 1 kg·m²·s^-2
^***) radiation source with frequency = 540×10¹² Hz and emitting energy = 1/683 J per second and per steradian

Base units

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