How to use it
- Choose what you know: radius, diameter, circumference or area.
- Type the value.
- The other three measurements update at once.
Formulas
| Measure | Formula |
|---|---|
| Diameter | d = 2r |
| Circumference | C = 2πr = πd |
| Area | A = πr² |
| Radius from C | r = C ÷ 2π |
| Radius from A | r = √(A ÷ π) |
Worked examples
- Radius 5. d = 10, C = 2π × 5 = 31.4159 and A = π × 25 = 78.5398.
- Circumference 2π. The radius is 1, the diameter 2 and the area π.
- Area 4π. The radius is 2 and the circumference 4π.
Tips
- Area units are squared: a radius in centimetres gives an area in square centimetres.
- Doubling the radius doubles the circumference but multiplies the area by four.
- For parts of a circle, such as a sector, use the area calculator.