What is the Circle Calculator?
Calculate the area, circumference and diameter of a circle from its radius.
Enter radius and the Circle Calculator returns area, circumference and diameter. Results update as you type, and nothing you enter leaves your device.
How it is calculated
Area = πr²
Circumference = 2πr
How to use it
- Enter radius.
- Read area in the results panel. It updates as you type.
- Change any value to compare scenarios, or use Share to send the result as a link.
Worked example
Common questions
What formula does the Circle Calculator use?
It uses: Area = πr²; Circumference = 2πr.
What do I need to use the Circle Calculator?
You need radius. Sensible starting values are filled in so you can see how it works straight away.
Is the Circle Calculator free, and is my data stored?
It is free with no sign-up. The calculation runs in your browser, so the values you enter are not sent to or stored on our server.