# Radius

The radius of a circle is the distance from its center to its circumference or from the center of a sphere to its surface. The radius is equal to half the diameter.

Radius is an elementary-school-level concept. It is listed in the California State Standards for Grade 4.

### Prerequisites

Circle: | A circle is the set of points in a plane that are equidistant from a given center point. |

Perimeter: | The perimeter is a length around the boundary of a closed two-dimensional region. The perimeter of a circle is called its circumference. |

Sphere: | A sphere is the set of all points in three-dimensional space that are located at a fixed distance from a given point. |