# Definition of the Radian Measure

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The radian measure is an angle measure in which an angle of 1 radian is counted as one. In other words, the radian measure of any angle is the ratio of this angle to the radian.

## The definition of the radian measure implies that the **magnitude of the round angle equals** **2π radians**.

There is also another way to determine the radian measure: the radian measure of an angle is the ratio of the arc length of the circle between the angle sides to the radius of this circle when the center of the circle coincides with the angle’s vertex.

# Formula of the Radian Measure of an angle

The Radian Length of angle α can be determined as follows: