Mathigon: Geometry: Circle and Pi: Degrees and Radians

Rather than dividing a circle into some number of segments (like 360 degrees), mathematicians often prefer to measure angles using the circumference of a unit circle - a circle with radius 1. This way of measuring angles is called radians, a unit for measuring the size of angles. One complete revolution (360 degrees) is 2 pi radians - the length of the circumference of a unit circle.