Circle Calculator – Find Radius, Circumference, & Area

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


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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 = d / 2

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.

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