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.
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 / 2π
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.