Number, called the circumference, that expresses the length of a circle.

### Formula

The formula to calculate the circumference of a circle is: *C* = 2π*r = π d.*

- The letter
*C*represents the circumference of the circle. - The letter
*r*represents the radius of the circle. - The letter
*d*represents the diameter of the circle. - The value of π is approximately 3.1416.

### Examples

- This circle has a radius of 3 centimetres.

*C* = 2π*r*

*C* ≈ 2 × 3.1416 × 3

*C* ≈ 18.8496 cm

- The circumference of the Earth is approximately 40 000 km.