Circle Calculator
Calculate a circle's area, circumference, diameter and radius from any one of them.
Everything about a circle from one measurement
A circle's radius, diameter, circumference and area are all linked, so knowing any one lets you find the rest. This calculates them all from whichever measurement you have, using the constant pi that ties them together.
Enter any one measurement to find the others.
Pi, the constant behind it all
Every circle relationship involves pi, the ratio of any circle's circumference to its diameter — a value of about 3.14159 that is the same for every circle, large or small, which is itself a remarkable fact. Pi is irrational, its decimals never ending or repeating, so any calculation uses an approximation, though a few decimal places suffice for almost any practical purpose. The relationships are worth knowing: the circumference is pi times the diameter, and the area is pi times the radius squared — note the area uses the radius squared, a common source of error when people use the diameter by mistake. From one measurement and pi, the whole circle is determined.
Questions & answers
What is pi?
The ratio of any circle's circumference to its diameter, about 3.14159 — the same for every circle, and irrational, so its decimals never end.
What is the most common circle-calculation mistake?
Using the diameter where the radius is needed, especially for area, which uses the radius squared — halve the diameter first.
Is anything uploaded?
No. The calculation happens in your browser.