Circle Calculator
Diameter, circumference and area from a circle's radius.
Quick answer
- Formula: Diameter = 2r; Circumference = 2πr; Area = πr²
- Worked example — Diameter: 10.0000 (full example below).
Circle Calculator
| Diameter | 10.0000 |
| Circumference | 31.4159 |
| Area | 78.5398 |
A radius of 5 gives a diameter of 10, a circumference of about 31.42 and an area of about 78.54 square units.
Everything about a circle flows from its radius and the constant π (pi ≈ 3.14159): the diameter is twice the radius, the circumference is 2πr, and the area is πr².
Enter the radius — the distance from the centre to the edge — and all three measurements are returned. Circles with the values here scale predictably: double the radius and the circumference doubles, but the area quadruples.
The formula
Worked example
A radius of 5 gives a diameter of 10, a circumference of about 31.42 and an area of about 78.54 square units.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
What exactly is pi (π)?
The ratio of any circle's circumference to its diameter — the same ≈ 3.14159 for every circle, whatever its size. It is an irrational number, so its decimal expansion never ends or repeats.
I know the diameter instead of the radius — what do I enter?
Halve it: the radius is diameter ÷ 2. A pizza described as 12 inches across has a radius of 6 inches, and therefore an area of about 113.1 square inches.
Category: Math calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
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