Circle Calculator

Enter a radius, a diameter or a circumference. The page fills in the other two lengths and the area, keeping length (unit) separate from area (unit²).

How it works

A circle is determined by one length. Diameter is 2r, circumference is 2πr, area is πr². Starting from circumference, r = C / (2π).

Area is not a length — a 5 cm radius gives a 10 cm diameter and a 25π cm² area. π is Math.PI.

Formula

d = 2r
C = 2πr
A = πr²
r = C / (2π)

Worked examples

From radius 5
r = 5 → d = 10, C = 10π, A = 25π
From circumference
C = 10π → r = 5

Useful notes

This is a planar circle, not a sphere. Sphere volume and surface area have their own pages.

FAQ

How do I find circumference from radius?: Multiply the radius by 2π. C = 2πr.
How do I find area from diameter?: Radius is half the diameter, then A = πr². A diameter of 10 gives r = 5 and area 25π.
Why are there two different units?: Radius, diameter and circumference are lengths. Area is length squared. They are not interchangeable.

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