# Circle calculator

> A circle is fully defined by a single measurement. Enter a radius, diameter, circumference, or area and this calculator returns all four properties.

Calculate circle radius, diameter, circumference, and area from any one known measurement.

**Interactive version:** https://onlinecalculator.me/math/circle/
**Category:** Math

## Formulas

| Property | Formula |
|---|---|
| Diameter | d = 2r |
| Circumference | C = 2πr = πd |
| Area | A = πr² |

## Example

Radius = 5:

- Diameter = 10
- Circumference = 2π × 5 ≈ 31.416
- Area = π × 25 ≈ 78.540

## Frequently asked questions

### What is the formula for the area of a circle?

Area = π × r². For a radius of 5, Area = π × 25 ≈ 78.54.

### What is the formula for circumference?

Circumference = 2πr = πd. It is the perimeter of the circle.

### How do I find the radius from the area?

Rearrange Area = πr² to get r = √(Area / π).

### How do I find the radius from the circumference?

Rearrange C = 2πr to get r = C / (2π).

### What units does this calculator use?

Whatever unit you enter the value in. If you enter 5 cm, all outputs are in cm (or cm²).

### How do I share my calculation?

Click "Share with my numbers" to copy a URL that restores your inputs.

## Sources

- [Circle — area (πr²) and circumference (2πr)](https://en.wikipedia.org/wiki/Circle) — Wikipedia
- [Circle — formulas and derivations](https://mathworld.wolfram.com/Circle.html) — Wolfram MathWorld

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