Quantity

Unit

Definition

Length

metre

m

The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

Mass

kilogram

kg

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Time

second

s

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Electric current

ampere

A

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10^-7 newton per metre of length.

Thermodynamic temperature

kelvin

K

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Amount of substance

mole

mol

Luminous intensity

candela

cd

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.