Radius and diameter: step-by-step example
27 October 2026
Understanding the words radius and diameter is a key part of circle vocabulary for the 11+ maths exam. If your child can spot the difference between these two terms, they will find circle questions much easier to tackle.
Example question
The radius of a circle is 4 cm. What is the diameter?
Step 1
Remember: the diameter is twice as long as the radius.
Step 2
Multiply the radius by 2.
4 cm × 2 = 8 cm
Step 3
The diameter is 8 cm.
Key insight: Once your child knows that the diameter is always twice as long as the radius, they can work out either value if they have the other. Now your child can recognise radius and diameter in circle questions and is ready to practise similar problems.
