Enter any one value — radius, diameter, circumference, or area — to calculate the rest.
Every measurement of a circle traces back to its radius through π: diameter is twice the radius (D=2R), circumference is 2πR, and area is πR². Enter any single one of the four values above, and the other three follow automatically.
π is defined as the ratio of a circle's circumference to its diameter — a constant that holds true for every circle, regardless of size. It's an irrational number (its decimal digits never terminate or repeat), which is why it's always approximated (as 3.14159… or 22/7) rather than written exactly.
Solve for radius first: R = √(A/π), then use that radius to find the diameter and circumference — exactly what this calculator does automatically.
Because all four measurements are mathematically linked through the radius — entering more than one risks contradictory inputs (a radius and area that don't actually match the same circle).
Pick another tool to jump straight to it.