The **radian** is the SI supplementary unit of angle. It is defined as the angle subtended at the center of a circle by an arc of circumference equal in length to the radius of the circle.

There are 2&pi (about 6.283185) radians in a complete circle. One radian is equal to 180°/π or approximately 57.29578°.

In calculus, the use of radians leads to the simple identity

Conversion of degrees to radians can be carried out using the formula