circumference

The circumference is the distance all the way around a circle, the circular equivalent of a perimeter. It is given by C = 2πr, where r is the radius — the distance from the center to the edge — and π (pi) is about 3.14. A circle with a radius of 7 cm has a circumference of 2 × 3.14 × 7 = 43.96 cm. Because the diameter is twice the radius, the same formula is often written C = πd. The unit is a length, so it is in plain centimeters or inches, never squared. The two common errors are dropping the 2 and computing πr, which is half the circumference, and squaring the radius; πr² is the area inside the circle, a different quantity measured in square units. Circumference measures the rim; area measures the surface.