National Physical Laboratory

How does the kilogram fit into the International System of units (SI)? (FAQ - Mass & Density)

The Système International d'unités - the SI system - is the coherent system of units adopted and recommended by the General Conference on Weights and Measures (CGPM). It is based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity.

Mass is therefore a base quantity in the SI and its unit is the kilogram, which is abbreviated to kg. The value of the kilogram is defined as being equal to the mass of the international prototype of the kilogram. The kilogram is unique in the SI inasmuch as it is the only unit whose definition is based on a physical artefact.

Last Updated: 17 Feb 2012
Created: 8 Oct 2007

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