Circle Calculator
By Santaji GadepatilinUpdated October 27, 2026
Area (cm²) vs Radius (cm) — other inputs held at your current values.
How to use this calculator
Type any one value — radius, diameter, circumference or area — and the other three are calculated. Change the unit of any field independently; area units (cm², m², ft²…) convert automatically.
The formulas
d = 2r C = πd = 2πr A = πr²
Working backwards:
- From circumference: r = C ÷ (2π)
- From area: r = √(A ÷ π)
Worked example
A circular table has a diameter of 1.2 m.
- Radius = 1.2 ÷ 2 = 0.6 m
- Circumference = π × 1.2 ≈ 3.77 m (the length of edge trim needed)
- Area = π × 0.6² ≈ 1.13 m² (the tabletop surface)
How area grows with size
Area depends on the square of the radius. Double the radius and the area becomes four times larger. That is why a 12-inch pizza has more than twice the area of an 8-inch pizza (113 in² vs 50 in²). The chart shows this curve.
Useful values
| Radius | Circumference | Area |
|---|---|---|
| 1 | 6.283 | 3.142 |
| 5 | 31.416 | 78.540 |
| 10 | 62.832 | 314.159 |
| 50 | 314.159 | 7,853.982 |
Frequently asked questions
What is the formula for the area of a circle?
Area = π × r², where r is the radius. A circle with a 5 cm radius has an area of about 78.54 cm².
How do I find the radius from the circumference?
Divide the circumference by 2π. A circumference of 31.42 cm gives a radius of 5 cm. Just type the circumference in the calculator.
What is π?
π (pi) is the ratio of any circle's circumference to its diameter, about 3.14159. It is the same for every circle.