- Pi x 2
**Pi x 1 x 1**- Pi x 0.5
- Pi squared

A circle’s radius is the length of a line drawn from its centre to any point on its circumference. Radii is the plural form (pronounced “ray-dee-eye”). Drag the orange dot around the circle in the diagram above to see how the radius remains constant at all points on the circle. A radius of a circle or sphere is any of the line segments from its middle to its perimeter in classical geometry, and it is also their length in more modern use. The name comes from the Latin radius, which refers to a ray as well as a chariot wheel spoke.