In the image above, the arc length, L, between A and B is the same length as the radius, R, when the angle AOB is 1 radian.

If we draw two straight lines that cross at the centre of a circle such that the length of the circumference between them (an arc) equals the radius, then the angle between these lines is 1 radian.

The angular circumference of a circle is 2π radians, or 360 degrees, so that:

• 1 radian = 360/2π degrees = 57^o 17’ 44.8”.