The radius of a circle is the distance from its center to any point on the edge. It is half the diameter, and it is the measurement every other circle formula is built from.
Math Dictionary / Radius
Radius
The radius is a line segment from the center of a circle to any point on the circle.
GeometryGrade 6Grade 7Grade 8GEDSATACT
What it means
The radius is half the diameter.
Also called: r
Examples
| If diameter = 10, radius = 5 |
| All radii in the same circle have equal length |
| Circle area uses r^2 |
Common mistake
The radius starts at the center, not at one side of the circle.
Try one
Question: If a circle has diameter 18, what is its radius?
Show answer
9.