# Circle Calculator

> Enter any one of a circle’s radius, diameter, circumference or area and get the other three, using π to full precision.

Category: Algebra, geometry & advanced math · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/circle-calculator

Every measurement of a circle follows from its radius, so knowing any one of radius, diameter, circumference or area gives you the rest.

Also called: Circumference calculator, Area of a circle calculator.

## Inputs
- I know the (options: Radius, Diameter, Circumference, Area; default radius)
- Value (default 5)

## Results
- Area
- Radius
- Diameter
- Circumference

## Formula

```
d = 2r;  C = 2πr = πd;  A = πr²;  r = C ÷ 2π = √(A ÷ π)
```

Where:
- r: radius
- d: diameter
- C: circumference
- A: area
- π: 3.14159265…

## Worked example
1. Diameter = 2 × 5 = 10.
2. Circumference = 2π × 5 = 10π ≈ 31.415927.
3. Area = π × 5² = 25π ≈ 78.539816.

**Result:** A circle of radius 5 has diameter 10, circumference 31.42 and area 78.54 square units.

## What the result means

Area grows with the square of the radius: doubling the diameter of a pizza gives four times as much pizza.

## Assumptions and limits
- A perfect circle in a flat plane.
- Results are rounded to 6 decimal places.

## Questions

### Why does 3.14 give a slightly different answer?

3.14 is π rounded to two decimals, about 0.05% low. This calculator uses π to double precision.

### How do I get the area from the diameter?

Halve the diameter to get the radius, then A = πr², or directly A = πd² ÷ 4.

## Related calculators
- [Surface Area Calculator](https://calculators.geekmidst.com/calculators/surface-area-calculator)
- [Concrete Calculator](https://calculators.geekmidst.com/calculators/concrete-calculator)
- [Length Converter](https://calculators.geekmidst.com/calculators/length-converter)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/circle-calculator