Calculate the circumference of a circle

The circumference is the distance all the way around the circle. The radius $r$ is the distance from the centre of the circle out to its edge.

The formula is:

$$C = 2 \cdot \pi \cdot r$$

You multiply the radius by 2 and then by $\pi$. Two times the radius is the same as the diameter, the distance straight across the circle. Pi is about 3.14, and it is exactly the ratio between circumference and diameter: the circumference is a little more than three times as long as the diameter.