Definitions of the SI Base Units

Length: metre (m)

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

Mass: kilogram (kg)

The kilogram is equal to the mass of the international prototype of the kilogram: a piece of platinum-iridium alloy kept at the International Bureau of Weights and Measures, Sévres, France.

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 cesium-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 x 10^-7 newton per metre of length.

Thermodynamic temperature: kelvin (K)

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

The unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature. In addition to the thermodynamic temperature (symbol T) there is also the Celsius (symbol t) defined by the equation t=T-T₀ where T₀=273.15 K. Celsius temperature is expressed in degree Celsius (symbol C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius. (The word degree and the sign ^o must not be used with kelvin or K).

Amount of substance: mole (mol)

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particle.
In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state.

Luminous intensity: candela (cd)

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