Circle Calculator
Find radius, diameter, circumference and area from whichever one you know.
Your result will appear here as soon as you add some input.
How to use it
- 1Enter your values in the fields above.
- 2The answer and the working appear immediately below.
- 3Check the working to see the formula with your numbers substituted.
Formulas used
- C = 2 x pi x r
- A = pi x r^2
About the Circle Calculator
Every circle measurement follows from the radius, so whichever value you have is converted to a radius first and everything else derived from it.
Doubling the radius quadruples the area but only doubles the circumference - the area depends on the square. That is why a 16-inch pizza is not twice a 12-inch one but about 1.8 times as much.
Questions people ask
Is your figures uploaded to a server?
No. This tool runs entirely in your browser using standard web APIs, so your figures never leaves your device. You can confirm it by opening your browser's network panel while the tool runs, or by disconnecting from the internet after the page has loaded.
Is it free, and do I need an account?
Yes, it is free, and there is no account, no sign-up and no watermark on the result. The project is funded by optional donations.