Definition
Circumference
Circumference is the distance all the way around a circle.
What it means
Imagine wrapping a string exactly once around a circle and then straightening the string. Its length is the circumference. Every circle has the same around-to-across ratio, so C = πd.
Remember it
Circumference goes around the circle; diameter goes across it.
Example
A circle has diameter 8 cm. Its circumference is C = πd = 8π cm, which is about 25.1 cm.
Watch for
Circumference is a length around a circle, not the area inside it.
Common mix-up
Use C = πd or C = 2πr; multiplying a radius by π alone gives only half the circumference.
Exact meaning
Circumference is the length of a circle’s boundary. It equals π times the diameter, or 2π times the radius, and is measured in linear units.