Circle Calculator

The area is pi times the radius squared (A = pi r squared). Enter the radius and this tool returns the area instantly; if you only know the diameter, circumference, or area, switch the known-measurement option and it works backward to the radius first.

Circumference equals 2 times pi times the radius (C = 2 pi r), which is the same as pi times the diameter. The calculator uses the full-precision value of pi, then rounds the displayed result.

Yes. Pick "Area" or "Circumference" as the known measurement. The radius is the square root of the area divided by pi, or the circumference divided by 2 pi, and the tool fills in every other value for you.

Yes. Every calculation happens locally in your browser. The numbers you enter are never uploaded or stored on a server.