Want to calculate the radius of a circle? Cool, try this calculator.

About This Calculator

This radius of a circle calculator will figure out the radius of any circle given the circumference, the diameter, or the area of the circle.

Terminology

"Area" - The amount of surface covered by a two dimensional shape. It is expressed in square units of measurement, for example square feet, square meters, square inches, square yards, etc.

"Circumference" - The total distance around the outside edge of a circle.

"Diameter" - The distance of a straight line that extends from edge to edge of the circle while passing through the center of the circle.

"Radius" - The distance of a straight line that extends from the center of the circle to the edge of the circle.

Calculating the Radius of a Circle Using Circumference

If you know the circumference of a circle, you can calculate the radius based on the following formula:

r = C / 2 ϖ

where

C = Circumference
ϖ = Pi = 3.14159265...
r = Radius

Calculating the Radius of a Circle Using Diameter

If you know the diameter of a circle, you can calculate the circumference based on the following formula. This one's easy, the radius is always half of the diameter:

r = d / 2

where

r = Radius
d = Diameter

Calculating the Radius of a Circle Using Area

If you know the area of a circle, you can calculate the radius based on the following formula: