SI derived unit

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The International System of Units (SI) specifies a set of seven base units from which all other units of measurement are formed. These other units are called SI derived units and are also considered part of the standard. SI units was after the French Le Système International d'Unités which opted for a universal, unified and self-consistent system of measurement units based on the MKS (metre-kilogram-second) system.

The names of SI units are always written in lowercase. The unit symbols of units named after persons, however, are always spelled with an initial capital letter (e.g., the symbol of hertz is Hz; but metre becomes m).


Derived units with special names

Base units can be combined to derive units of measurement for other quantities. In addition to the two dimensionless derived units radian (rad) and steradian (sr), 20 other derived units have special names.

Other common units, such as the litre, are not SI units, but are accepted for use with SI (cf. non-SI units accepted for use with SI).

Supplementary units

Until 1995, the SI classified the radian and the steradian as supplementary units, but this designation was abandoned and the units were grouped as derived units.

Other quantities and units

See also


  • I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd edition (June 1993), Blackwell Science Inc (p. 72)

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