Radius

For the human bone, see Radius (bone).

The radius of a circle

In geometry, the radius of a circle or sphere, written as $r$, is the shortest connection between the center and the boundary. It is either the distance from the center to the perimeter (in the case of a circle), or the distance from the center to the surface (in the case of a sphere). It is half of the diameter. That is, when r is the radius and d is the diameter:

r = d ÷ 2

d = 2 x r = r + r

The relationship between the radius ${\displaystyle r}$ and the circumference ${\displaystyle c}$ of a circle is $~c=2\pi r.$

The area ${\displaystyle A}$ of a circle of radius ${\displaystyle r}$ is ${\displaystyle ~A=\pi r^{2}.}$

Related pages

• Area
• Covalent radius
• Diameter
• Radian

Other websites

• Definition and properties of the radius of a circle with interactive applet