Circle Calculator
Calculate a circle's area, circumference and diameter from its radius.
From a circle's radius or diameter, you can derive everything else that matters — area, circumference, and the other measurement — in one step.
Radius, diameter, and π working together
Diameter is twice the radius. Area uses π × radius². Circumference uses 2 × π × radius. Since π is a fixed constant, knowing just one measurement is enough to derive all the others.
Frequently asked questions
Why is π an irrational number?
Its decimal digits continue infinitely without repeating — a proven mathematical property of the circle.
How precise should I be with π for real-world projects?
For construction or DIY, 3.14 or 3.14159 is more than sufficient precision.