11+ Maths: Calculating Perimeter (GL & CEM style)
21 October 2026
Understanding perimeter
Perimeter is the total length around the edge of a shape. It is a key topic in 11+ maths, especially in GL and CEM style exams.
Children need to be confident with perimeter to solve questions about rectangles, compound shapes, and word problems.
How to calculate perimeter
To find the perimeter, add the lengths of all the sides together. For rectangles and squares, you can use a formula to make this quicker.
Worked example
A rectangle has a length of 5 cm and a width of 4 cm.
Add the lengths of all sides: 5 cm + 4 cm + 5 cm + 4 cm.
Or use the formula: 2 × (5 cm + 4 cm) = 2 × 9 cm = 18 cm.
The correct answer is 18 cm.
Key insight: count every side
It is important to include every side when calculating perimeter. For compound shapes, work your way around the outside, adding each edge only once.
A tip for practising at home
Try measuring and adding the sides of everyday objects or drawing shapes together. Practising with different shapes helps children recognise patterns and build confidence.
