Definition: Circumference is the distance around a circle, equal to pi times the diameter, or C = 2 pi r.
What is circumference?
It is the perimeter of a circle, the length of the curve if you straightened it out.
What is the formula?
C = pi x d, or equivalently C = 2 x pi x r, where d is the diameter and r is the radius. Pi is about 3.14, or 22/7 as a rough fraction.
Why does pi appear?
For every circle, the circumference divided by the diameter equals the same number, pi. A circle with diameter 1 has circumference about 3.14.
How do you use it?
A bike wheel has radius 13 inches. Then C = 2 x 3.14 x 13 = 81.64 inches. In one full turn, the wheel rolls 81.64 inches. To go 100 feet, which is 1,200 inches, it spins about 1,200 / 81.64, roughly 14.7 times.
How can you get the diameter from C?
Divide: d = C / pi. A circular track of 400 meters has diameter about 400 / 3.14, roughly 127.4 meters.
What is a common mistake?
Using the radius in C = pi x d. If you only know r, double it first.