📖 Math Dictionary

Radius

/RAY-dee-us/

Definition

The radius is the distance from the center of a circle to any point on its edge. It is exactly half the diameter. All radii (plural) of a circle are equal in length.

💡Example

If a circle has a diameter of 10 cm, its radius is 10 ÷ 2 = 5 cm. The circumference formula C = 2πr uses the radius. Area formula A = πr² also uses radius.

🎓Used in Grades

Grades 4-10KS2 to GCSEÅrskurs 4-9

✏️Practice Checks

A circle has diameter 18 cm. What is its radius?

Answer: 9 cm

The radius is half the diameter: 18 / 2 = 9.

A circle has radius 6 cm. What is its diameter?

Answer: 12 cm

The diameter is twice the radius: 2 x 6 = 12.

Which formula uses radius to find circle area?

Answer: A = pi r^2

Square the radius and multiply by pi.

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🔗Related Terms

Diameter

📚Learn More

circles →area of circle →circumference →

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