# Circumference

The
**
circumference
**
of a circle is the distance around the outside.

If you know the radius $r$ , you can calculate the circumference $C$ using the formula

$C=2\pi r$ ,

where $\pi $ is pi , an irrational number approximately equal to $3.14$ .

The formula is sometimes written

$C=\pi d$ ,

where $d$ is the diameter .