Divide Circumference by pi (3.1415) to get diameter. Divide diameter by 2 to get radius.

if you are given any one of the following, radius, diameter, circumference, or area. You can find all of the others. Memorize the formulas for Circumference (Pi*diameter) and Area (Pi * Radius squared) and the fact that radius * 2 = diameter, and you can do all of them. so: