Radius → area & circumference

From radius: area πr² and circumference 2πr (any consistent unit).

Radius (same unit for area output²)

FAQ

Which π does the page use?

JavaScript’s built‑in Math.PI: area = πr² and circumference = 2πr in the same length unit as the radius you enter (six decimals on output).

Does the decimal separator matter?

This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.