# Geometry » Circles

## What is a circle?

The definition of a circle is:

A set of points in a plane that are equidistant to a given point, the centre.

The blue line in the drawing below is the circle. So not the area within.

The distance between the centre and the circle is called the `radius`*^{1}.

The distance circle-circle through the centre is called the `diameter`.

The perimeter of the circle is called the `circumference`.

## Calculating with circles

`diameter` = 2 × `radius`

`radius` = 12 × `diameter` (or `diameter` : 2)

`area circle`*^{2} = `radius` × `radius` × `π`

`circumference` = `diameter` × `π`

In letters and as short as possible:

`d` = 2`r`

`r` = 12`d`

`A` = `π``r ^{}`

^{2}

`C`=

`d`

`π`

*^{1} | The plural form of radius is radii. |

*^{2} | With the area of the circle, we mean the area enclosed by the circle. The area within the circle is actually called a disc. |