Circle Calculator

Enter any one known measurement of a circle to calculate the other three.

How to Use the Circle Calculator

Enter any single known measurement — radius, diameter, circumference, or area — and the other three fill in from it, rather than needing radius specifically every time.

Radius, diameter, circumference, area — know any one and the other three follow. This Circle Calculator takes whichever measurement you happen to have.

diameter = 2 × radius
circumference = 2 × π × radius
area = π × radius²

Whatever you enter, the calculator solves for radius first and derives everything else from there — working backward from circumference means radius = circumference ÷ (2π); from area, radius = √(area ÷ π).

π itself never terminates or repeats, so technically every calculation using it is an approximation at whatever precision you stop at. This tool runs full floating-point precision, which is overkill for anything short of aerospace engineering — the rounding error you'd actually see is smaller than your tape measure.