Worked example
27 October 2026
Understanding the key words used to describe parts of a circle helps children feel confident with circle questions. The two most important terms are 'radius' and 'diameter'. The radius is the distance from the centre of the circle to the edge. The diameter is a straight line going all the way across the circle, passing through the centre.
Worked example
A circle has a radius of 5 cm. What is its diameter?
Step 1: The diameter is twice as long as the radius.
Step 2: Multiply the radius by 2.
5 cm × 2 = 10 cm
The diameter is 10 cm.
Key insight: Once your child knows the radius, they can always find the diameter by doubling it. This is a common type of question in 11+ maths. Now your child can spot radius and diameter in any circle question, and is ready to try similar practice problems themselves.
