Circumference of a Circle

Circumference of a Circle

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

Formula: Using Radius

C = 2*pi*r. C: Circumference of a circle, pi: 3.141592..., r: Radius of a circle

C = 2πr

C: Circumference of a circle
π = 3.141592...
r: Radius of a circle

π is a constant number
(It's an irrational number.)

Example 1

Find the circumference of the given circle. Radius: 5

r = 5

So C = 2⋅π⋅5.

2⋅5 = 10

So C = 10π.

Formula: Using Diameter

C = pi*d. C: circumference of a circle, pi: 3.141592..., d: diameter of a circle

C = πd

C: Circumference of a circle
π = 3.141592...
d: Diameter of a circle

d = 2r

So C = π⋅2r
= πd.

Example 2

Find the circumference of the given circle. Diameter: 13

d = 13

So C = π⋅13
= 13π.