Circle Calculator

Enter any one circle measurement and get the radius, diameter, circumference and area straight away.

Circle calculator

Which value do you know?

Radius: —

Diameter: —

Circumference: —

Area: —

Uses pi = 3.141592653589793 and rounds every result to six decimal places.

Common questions about the free online circle calculator.

Enter the radius and read the area, which is pi multiplied by the radius squared. The other values appear at the same time.

Choose the radius or diameter mode and the circumference is calculated as pi times the diameter, shown to six decimal places.

The radius is the distance from the centre to the edge, and the diameter is twice that, running right across the circle.

The calculator uses the full double precision value of pi, 3.141592653589793, so the results stay as accurate as possible.

Yes. Select the area mode, enter the area, and the radius, diameter and circumference are all derived from it.

Results are rounded to six decimal places so they are easy to read while staying accurate enough for most work.

No. Everything is calculated locally in your browser and nothing you type is sent anywhere.

Yes. The circle calculator is fully responsive and works on phones, tablets and desktop browsers.

Yes, the circle calculator is completely free with no account and no limit on how many times you use it.

Last updated: October 10, 2026