Circle Calculator – Find Radius, Circumference, & Area

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



Trying to solve the dimensions of an ellipse? Try our ellipse calculator.

How to Calculate Circle Radius

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

diagram of a circle showing the radius, diameter, and circumference

If the Diameter is Known

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

r = d2

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

If the Area is Known

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 the Circumference is Known

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

r = C

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

How to Calculate the Area of a Circle

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.

How to Find the 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.