Circle Calculator - Find Radius, Circumference, & Area

Solve the radius, diameter, area, and circumference of a circle using the calculator below.

How to Calculate Circle Radius

There are several ways to find the radius of a circle.

The radius of a circle is equal to half the circle’s diameter. The following formula defines the radius

r = d / 2

The radius r is equal to the diameter d divided by 2.

If the area of a circle is known, then use the inverse of the area formula to find the radius.

r = A ÷ π

The radius r is equal to the square root of the area A divided by pi.

If a circle’s circumference is known, then use the inverse of the circumference formula to solve the radius.

r = C / 2π

The radius r is equal to the circumference C divided by 2 times pi.

The area of a circle can be found using a simple formula:

A = πr²

The area of a circle A is equal to pi times the radius r squared.

You can also use our circle area calculator to easily solve for area using the radius, diameter, or circumference.

The circumference of a circle can be solved using the formula:

C = 2πr

The circumference of a circle C is equal to 2 times pi times the radius r.

You can also use our circumference calculator to solve using the radius, diameter, or area.