# Steradian

## A unit of solid-angle measure in the International System of Units (SI)

The **steradian** is a unit of solid-angle measure in the International System of Units (SI). It is defined as

- the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere’s radius.

The complete surface area of a sphere is 4π times the square of its radius and

- the total solid angle about a point is equal to 4π steradians.