There are two other important distances on a circle, the radius (r) and the diameter (d). The radius, the diameter, and the circumference are the three defining aspects of every circle. Given the radius or diameter and pi you can calculate the circumference. The diameter is the distance from one side of the circle to the other at its widest points. The diameter will always pass through the center of the circle. The radius is half of this distance. You can also think of the radius as the distance between the center of the circle and its edge.
This diagram shows the circumference, diameter, center, and radius on a circle.
