Circle Calculator

A circle calculator derives a circle's key measurements — area, circumference and diameter — from just its radius, using the mathematical constant π (pi), the fixed ratio of any circle's circumference to its diameter. These measurements come up constantly in construction (pipe and tank sizing), engineering, and everyday design work.

Enter the circle's radius. The calculator finds the area as π × radius², the circumference as 2 × π × radius, and the diameter as simply twice the radius — all three derived from that one measurement.

Area = πr²   Circumference = 2πr