What is a Radius?
A line segment that joins the center of a circle with any point on its circumference. The plural is radii.
In a circle, the radius is the distance from the center of the circle to any point on its circumference.
Every point on the circle is the same distance from the center, so all radii in a circle are equal. For example:
- If a circle’s radius is 5 cm, then every point on the circle is exactly 5 cm away from its center.
- The diameter of a circle is twice its radius. So if the radius is 5 cm, the diameter is 10 cm.
The radius is important for calculating:
- Diameter (diameter = 2 × radius)
- Circumference (C = 2 × π × radius)
- Area (A = π × radius²)
When Do Students Learn About Radius?
Students first encounter the concept of radius when they explore basic circle parts in geometry.
Grades 3–5 – Introduction to Radius
Students learn what a radius is and how it connects the center of a circle to its circumference.
Grades 6+ – Using Radius in Formulas
Students use radius to calculate diameter, circumference, and area, and apply it in coordinate geometry and word problems.
