Circle calculator
Radius, diameter, circumference and area of a circle from any one of them.
About this calculator
This circle calculator finds every measurement of a circle from one you already know: the radius, the diameter, the circumference or the area.
It's handy for garden beds, round tables, pipes and wheels, and for checking homework.
How it works
The diameter is twice the radius, the circumference is 2 × π × radius, and the area is π × radius². From any one of them the calculator works back to the radius, then finds the others.
Last reviewed:
Worked example
- I know the: Radius
- Value: 5
Area: 78.5398163397
Common questions
What is the formula for the circumference of a circle?
C = 2πr, or π times the diameter. A circle with a radius of 5 has a circumference of about 31.42.
How do I find the radius from the area?
Divide the area by π and take the square root: r = √(A ÷ π).
What is pi?
The ratio of a circle's circumference to its diameter, about 3.14159. It never ends or repeats, so the calculator uses it to full precision.
The story behind it
Where the idea came from: Around 250 BC, Archimedes showed that pi lies between 3 10/71 and 3 1/7 by fitting polygons inside and outside a circle.
Archimedes used polygons with up to 96 sides to trap the value of pi, a method that stayed the best for centuries. Today pi has been calculated to trillions of digits, though a handful is enough for almost any real job.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
Meet the founder