This circle calculator works from any single property — radius, diameter, circumference or area — and instantly returns the other three. It is handy for geometry homework, engineering, crafts and construction, where you often know one measurement and need the rest. It is unit-agnostic: enter a value in any unit and lengths come back in that unit, with area in that unit squared.
How it works
The calculator first derives the radius from whatever you supply, then computes everything else from it:
| Known value | Radius from it |
|---|---|
| radius (r) | r |
| diameter (d) | d ÷ 2 |
| circumference (C) | C ÷ (2π) |
| area (A) | √(A ÷ π) |
Once it has the radius, it returns diameter = 2r, circumference = 2πr and area = πr². It uses JavaScript’s full-precision Math.PI (3.141592653589793) before rounding for display.
Example
Starting from a radius of 5:
diameter = 2 × 5 = 10 circumference = 2π × 5 ≈ 31.415927 area = π × 5² ≈ 78.539816
So a circle of radius 5 has a diameter of 10, a circumference of about 31.42, and an area of about 78.54 square units. Everything runs in your browser — nothing is uploaded.