Circle Calculator
Area, circumference, and diameter of a circle from its radius.
Worth Knowing
Every quantity here flows from the radius through the constant π, the ratio of any circle's circumference to its diameter. π is irrational — its decimals never end or repeat — so every area or circumference is technically an approximation, however many digits you carry. A subtle point: because area depends on the radius squared, doubling a circle's radius quadruples its area, which is why a 16-inch pizza has far more than twice the food of a 8-inch one.
How to Use
- Enter Radius into the Circle Calculator above.
- Your result appears instantly as you type — there's no submit button or page reload.