National Physical Laboratory

SI Base Units

The SI (Système International d'Unités) is a globally agreed system of units, with seven base units.

Formally agreed by the 11th General Conference on Weights and Measures (CGPM) in 1960, the SI is at the centre of all modern science and technology. The definition and realisation of the base and derived units is an active research topic for metrologists with more precise methods being introduced as they become available.

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  • The ampere is the SI base unit relating to electrical quantity.
  • The candela is the SI base unit relating to luminous intensity.
  • The kelvin is the SI base unit of thermodynamic temperature.
  • The kilogram is the SI base unit of mass.
  • 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.
  • 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.
  • 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.


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