Circle: Circumference

How to find the circumference of a circle: 2 formulas, 2 examples, and their solutions.

Formula

Formula

C = 2πr

C: Circumference of a circle
r: Radius

If a radius is given,
use this formula.

C = πd

d: Diameter

If a diameter is given,
use this formula.

Example 1

Example

Solution

r = 5
So use C = 2πr.

C = 2π⋅5

2π⋅5 = 10π

So 10π is the answer.

Example 2

Example

Solution

d = 13
So use C = πd.

C = π⋅13

π⋅13 = 13π

So 13π is the answer.