Circle Calculator
Enter any one circle measurement — radius, diameter, circumference or area — to get all the others.
How to use it
- Choose what you know Radius, diameter, circumference or area.
- Enter its value Any positive number.
- Read the rest The other three are computed at once.
Examples
| radius 5 | area ≈ 78.54 |
|---|---|
| area π | radius 1 |
About this tool
Every measurement of a circle is linked to every other through π, so knowing just one lets you find them all. This calculator takes whichever value you have — radius, diameter, circumference or area — and returns the complete set.
It’s handy for geometry homework, DIY and design work where you might know the distance around something but need its area, or vice versa. The maths runs entirely in your browser, so results are instant.
Frequently asked questions
What formulas does it use?
It uses the standard relationships: circumference = 2πr, area = πr², and diameter = 2r, inverting them as needed.
What units does it use?
Whatever unit you enter. Lengths come back in that unit and the area in its square, so a radius in cm gives an area in cm².
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Updated June 13, 2026