# Area of a Circle Calculator

URL: https://speedycalculator.com/calculators/math/area-of-a-circle/
Last updated: 2026-10-30

> The area of a circle calculator finds the area of a circle from its radius, diameter or circumference, and works backwards from an area to the radius.

## Inputs

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

## Results

- Area
- Diameter
- Circumference
- Area in terms of π (number × π)

## Formula

```
A = πr² = πd²/4 = C²/(4π)
```

## Explanation

The area of a circle is π times the radius squared. You need it to size pizza (a 16-inch pizza has 78% more area than a 12-inch one), to buy circular rugs or turf, to find pipe cross-sections and to work out the pressure on a piston.

### How to use the Area of a Circle Calculator

- **Radius r**
- Every result field is editable: type into **Area** or any other result and **Radius r** plus every other field update to match.

Results update as you type, and everything is calculated in your browser.

### How it works

Enter the radius or type into the diameter or circumference field. Changing the area field finds the radius that gives that area.

### What each result means

- **Area** = pi×[Radius r]^2
- **Diameter** = 2×[Radius r]
- **Circumference** = 2×pi×[Radius r]
- **Area in terms of π (number × π)** = [Radius r]^2

### Example values

How the main result changes when **Radius r** changes and everything else stays at the example values above:

Radius r (cm)Area (cm²)2.519.6353.7544.1786578.53986.25122.71857.5176.714610314.1593

## Worked example

Inputs — Radius r: 5 cm.

Results — **Area = 78.539816 cm²**; Diameter = 10 cm; Circumference = 31.415927 cm; Area in terms of π (number × π) = 25 π.

## Key terms

- **Area**: The space inside the circle.

## FAQ

**What is the area of a circle with a 10 cm diameter?**
π × 5² ≈ 78.54 cm².

**Why πr²?**
Cutting a circle into thin wedges and rearranging them gives a near-rectangle of sides πr and r.

**How do I find the radius from the area?**
r = √(A/π).

**Which is bigger: one 16" pizza or two 10" pizzas?**
One 16" (201 in²) beats two 10" (157 in²).


## Sources

- [OpenStax – Prealgebra 2e](https://openstax.org/details/books/prealgebra-2e)
- [OpenStax – Algebra and Trigonometry 2e](https://openstax.org/details/books/algebra-and-trigonometry-2e)
- [Wolfram MathWorld – Circle](https://mathworld.wolfram.com/Circle.html)