# Circumference of Circle

The **circumference of a circle** is a length of an around arc. How to imagine the **circumference of a circle**? For example, you have a tire and make a point on its edge. After one round move, the point goes a distance which equals to the **circumference of a circle**.

To calculate the **circumference of a circle** we need to know its diameter or its radius and the pi number (Ludolph's number). The pi number is a constant and equals to 3.141592... In a circle, the diameter is d = 2 × radius "r". The elementary unit is one metre (1 m).

d=2 × r, also r = d / 2

## Circumference of Circle - formula

**Circumference of Circle** = Ludolph's number × diameter

or

**Circumference of Circle** = 2 × Ludolph's number × radius

C = π × d = 2 × π × r