Circumference
The distance around the outside of a circle.
Circumference = 2 × π × radius = π × diameter. Pi (π) is approximately 3.14159. A circle with a 10-inch diameter has a circumference of 31.4 inches. This formula is used constantly in engineering, construction, and everyday life — calculating wheel rotations per mile, pipe sizing, belt lengths, and track distances. A standard outdoor running track is 400m in circumference at lane 1.