Perimeter of Triangles
23 October 2026
Understanding how to find the perimeter of triangles is an important skill for 11+ maths. If your child can add up the sides of a triangle, they can solve these questions with confidence.
Let's look at a clear example to see how it works in practice.
Worked example
A triangle has side lengths 5 cm, 7 cm and 9 cm. What is its perimeter?
Write down the lengths of all three sides: 5 cm, 7 cm, and 9 cm.
Add the lengths together: 5 + 7 + 9.
Work out 5 + 7 = 12.
Now add 12 + 9 = 21.
The correct answer is 21 cm.
Now your child can spot perimeter questions for triangles and knows exactly what to do. Practise more questions just like this to build speed and accuracy.
Key insight: Perimeter questions always ask for the total length around the shape, so adding every side is the key step.
