# SI base unit

The SI system of units defines seven SI base units: fundamental physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. Use SI prefixes to abbreviate long numbers.

## No circular references

Note that there are two base units above which do not appear to be defined purely in terms of other base units.

1. The ampere is defined in terms of newtons. However, one newton is 1 kg · m · s-2.
2. The candela is defined in terms of hertz, watt and steradians. One hertz is 1 s-1, one watt is 1 J · s-1 = 1 kg · m2 · s-3 and the steradian is the dimensionless solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2.

Therefore there are no circular references in the definition of the base units.

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