Circumference Calculator

Find the distance around a circle from its diameter, radius or area, or work back from a measured circumference to the diameter.

Last updated: October 22, 2026How this is calculated →

The circumference of a circle is pi times its diameter, or two times pi times its radius. A circle 10 in across measures 31.42 in around and encloses 78.54 square inches. Enter a diameter, radius, circumference or area and the calculator gives the other three.

Results assume a perfect circle and are rounded to two decimals. The calculator does not account for wall thickness: a tape wrapped around a pipe or tank measures its outside, so the inside diameter is smaller than the figure shown.

I know the

Diameter (in)

Unit

Circumference

31.42 in

Area: 78.54 square in
Radius: 5 in
Diameter: 10 in

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About the Circumference Calculator

Pi is the ratio of any circle's circumference to its diameter, so the distance around is always a little over three times the distance across. That makes the formula C = pi d, or C = 2 pi r when you start from the radius. A 12 in pizza has a crust 37.7 in long, and a bike wheel with a 26 in diameter rolls 81.68 in on each turn. Going the other way is often the practical case: a diameter is hard to measure on a tree trunk, a pipe or a column, but a tape wrapped around it gives the circumference directly. A trunk that tapes at 75 in has a diameter of 23.87 in. Archimedes showed that the area equals a right triangle with the radius and the circumference as its legs, which is where A = pi r squared comes from; the calculator uses it to turn an area back into a size, so 100 square cm of circle has a radius of 5.64 cm.

Frequently asked questions

How do I find the circumference from the diameter?+

Multiply the diameter by pi, about 3.14159. An 18 in diameter gives 56.55 in. With a radius, double it first or use 2 times pi times r; a 5 cm radius gives 31.42 cm.

How do I find the diameter from the circumference?+

Divide the circumference by pi. A circumference of 100 cm is a diameter of 31.83 cm and a radius of 15.92 cm.

Can I get the circumference from the area?+

Yes. Divide the area by pi and take the square root to get the radius, then multiply by 2 times pi. An area of 50 square cm gives a radius of 3.99 cm and a circumference of 25.07 cm.

How much farther does a bigger wheel travel per turn?+

Circumference grows in step with the diameter, so every extra inch across adds about 3.14 in to each turn. Going from a 26 in to a 29 in wheel adds 9.42 in of travel per revolution.

Formulas: Archimedes, Measurement of a Circle, Proposition 1; Weisstein EW, "Circle," MathWorld.

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<a href="https://manytools.co/math/circumference-calculator">Circumference Calculator by ManyTools</a>