Radians:

On the initial side of angle q, let P be a point one unit away from the vertex.
If q ³ 0, let s be the arc length traced by P as the initial side rotates through q; If q is negative , then the arc length s negative, then the radian measure of q is s.

q = s rad

If q is a complete rotation then q = 2p rad.

180° = p rad