# How to Find the Circumference of a Circle

The circumference or the perimeter of the circle can be found by the formula:

C = 2πr

where r is radius of the circle, π (pi number) is the mathematical constant ≈ 3.141592653589793

## Example nr. 1:

x^{2} + y^{2} = 5 => x^{2} + y^{2} = (√5)^{2}

r = √5 => C = 2πr = 2π√5 = 2√5π

C = 2√5π ≈ 2√5 * 3.141592653589793

## Example nr. 2:

Let diameter of the circumference is D = 24, then find C - ?

C = 2πr = C = 2π

D2

= πD = 24πC = 24π

## Example nr. 3:

Let C = 30π, then find D - ?

C = 2πr => 2πr = 30π

2r = 30

D = 30

Note: Don't forget that C = 2πr =>

Cr

= 2π = const for every circumference of a circle.