Q:
# What is a radius?

**The radius of a circle is the length of a line segment from its center to its outer edge.** The same definition also applies to the radius of a sphere. Alternatively, a shape's radius can be defined as half of its diameter. The diameter is the length of a straight line segment passing through a circle or sphere's center.

Credit:
ConstantinosZ
iStock / Getty Images Plus
Getty Images

The radius is commonly used in many areas of geometry. For example, the radius is required to calculate the area of a circle or sphere, as well as the volume. For shapes that do not have an obvious center, the word "radius" can refer to its circumradius, meaning the radius of a circle that passes through all vertices. In circles and spheres, the radius equals the diameter when multiplied by two. Measurements relating to diameter and radii are in linear units, such as inches and centimeters.

The word originates from Latin "radius," meaning the spoke of a chariot wheel. The plural form of "radius" can be either the Latin "radii" or the conventional English "radiuses." The typical mathematical variable name and abbreviation for radius is "r." Occasionally, the word "radius" is used to refer to the line segment itself rather than the length.

Learn more about Geometry-
Q:
## How do you measure diameter?

A: If the radius of the circle is known, the diameter formula is diameter=2R, where R equals the circle's radius. If the circumference of the circle is known,... Full Answer >Filed Under: -
Q:
## How do you calculate circumference?

A: The circumference of a circle is calculated by multiplying pi by the diameter, or 2 pi times the radius. The diameter or radius value is required to calcul... Full Answer >Filed Under: -
Q:
## What is the radius of a 10-foot circle?

A: A circle that measures 10 feet across has a radius of 5 feet. The diameter is the distance from one side of the circle to the other, passing through the ci... Full Answer >Filed Under: -
Q:
## How do you find the area of a sector of a circle?

A: The equation for the area of a circle is pi times the square of the radius or A = πr2. Calculating the area of a sector involves figuring out what fraction... Full Answer >Filed Under: