Radians are a unit that measures angle as the ratio of the angle’s arc length over the radius of a circle. For example, the angle (theta) is shown below and can be measured by dividing the arc length by its corresponding radius that forms the angle.

Radians Definition

In the radian angle system, a full rotation is equal to (tau) radians, where tau is defined as the circumference of a circle divided by its radius.

Related Terms


Degrees are a unit of measurement for angles, representing the rotation between two rays. The degree angle system divides a full rotation into 360 units called degrees.


An angle is a measure of rotation or turning and is usually measured in degrees or radians. In geometry, an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex. The size of an angle is determined by the amount of rotation from one ray to the other.