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SI derived units are part of the SI system of measurement units and are derived from the seven SI base units.

## Dimensionless derived units Edit

The following SI units are actually dimensionless ratios, formed by dividing two identical SI units. They are therefore considered by the BIPM to be derived. Formally, their SI unit is simply the number 1, but they are given these special names, for use whenever the lack of a unit might be confusing.

Dimensionless SI units
Name Symbol Quantity Definition
radian rad Angle The unit of angle is the angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are $2\pi$ radians in a circle.
steradian sr Solid angle The unit of solid angle is the 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. There are $4\pi$ steradians on a sphere.

## Derived units with special names Edit

Base units can be put together to derive units of measurement for other quantities. Some have been given names.