# Circle Calculator

URL: https://speedycalculator.com/calculators/math/circle/
Last updated: 2026-10-25

> The circle calculator returns the area, diameter and circumference of a circle from its radius, using A = πr² and C = 2πr. You can also type a known area to work backwards to the radius, with free choice of length and area units.

## Inputs

- Radius (m, cm, mm, km, in, ft, yd, mi)

## Results

- Area — Type an area to find the radius.
- Diameter
- Circumference

## Formula

```
A = π r²
C = 2 π r
d = 2 r
```

## Explanation

Every measurement of a circle follows from its radius. Knowing any one value lets you find all the others.

### How to use it

Enter the radius. If you know the diameter, halve it first. If you know the area, type it into the result and the radius is solved.

### Everyday uses

Size a round table cloth, compare pizza sizes (a 16-inch pizza has about 1.8 times the area of a 12-inch one), or work out how much seed covers a circular lawn.

### What each result means

- **Area** = pi × [Radius]^2
- **Diameter** = 2×[Radius]
- **Circumference** = 2×pi×[Radius]

## Worked example

Radius 5 m.

Area = π × 5² ≈ **78.54 m²**. Circumference = 2π × 5 ≈ 31.42 m. Diameter = 10 m.

## Key terms

- **Radius**: Distance from the centre to the edge.
- **Diameter**: Distance across the circle through the centre.
- **Pi (π)**: The ratio of circumference to diameter, about 3.14159.

## FAQ

**How do I get radius from circumference?**
r = C ÷ 2π.

**Why does area grow faster than diameter?**
Area depends on the square of the radius, so doubling the diameter quadruples the area.

**Can I use different units?**
Yes, choose length and area units independently.

**What formula does the Circle Calculator use?**
A = π r².

**Can I work backwards from the result?**
Yes. Type the result you want into Area and the calculator finds the matching radius.


## Sources

- [Wolfram MathWorld – Circle](https://mathworld.wolfram.com/Circle.html)