Circle Area and Circumference Calculator

What this calculator does

This tool finds the area, circumference, and diameter of a circle from a single input: its radius. It works with any consistent unit of length (inches, centimeters, meters, and so on) and returns results in the matching squared unit for area.

The formulas used

Area is calculated as A = πr², and circumference (the distance around the circle) is calculated as C = 2πr. Diameter is simply twice the radius. These are the standard formulas taught in geometry, using π (approximately 3.14159).

Good to know

Make sure your radius value uses the unit you want your answer in — the calculator does not convert between units. This tool is intended for general educational and planning purposes.