The measure of an angle is determined by the amount of rotation from the initial side to the terminal side.  In radians, one complete counterclockwise revolution is $2\pi$ and in degrees, one complete counterclockwise revolution is $360°$ .  So, degree measure and radian measure are related by the equations

and

From the latter, we obtain the equation .  This leads us to the rule to convert degree measure to radian measure.  To convert from degrees to radians, multiply the degrees by .

Example 1:

Convert $60°$ to radian measure.

Example 2:

Convert $150°$ to radian measure.