The circumference is the length of the outer edge of a circle — its perimeter. Knowing this measurement is essential when you need to figure out how much material goes around a circular object: round frames, tank lids, wheels, or running tracks.

The Formula

C = 2 × π × r

• C — circumference length (cm, m, etc.)
• π — the number pi, approximately 3.14159
• r — radius: distance from center to edge

The equivalent formula using the diameter (d = 2r) is C = π × d. This is actually how π was originally defined: the ratio of the circumference to the diameter of any circle.

Step-by-Step Example