Radian
A unit for measuring angles. 180° = π radians,
and 360° = 2π radians.
The number of radians in an angle equals the number of radii it
takes
to measure a circular arc described by that angle.
Note: 360° equals 2π radians
because a complete circular arc has length equal to 2π times
the radius.

Formula: θ = s/r
θ = measure of the central angle in radians
s = arc length
r = radius of the circle


Example:
s = 10
r = 5
θ = 10/5 = 2 radians 
Commonly used angles in degrees and radians
See also
Special angles
