11+ Maths: Calculating Perimeter (GL & CEM style)
21 October 2026
What is perimeter
Perimeter is the total distance around the outside of a two-dimensional shape. In 11+ maths, children are often asked to find the perimeter of rectangles, squares and other shapes.
Understanding perimeter helps children solve real-world problems, such as working out the length of fencing needed for a garden.
How to calculate perimeter of a rectangle
To find the perimeter of a rectangle, add the length and width together, then multiply by 2. This method works for any rectangle, no matter the measurements.
Worked example
Length = 5 cm
Width = 3 cm
First, add the length and width:
5 cm + 3 cm = 8 cm
Next, multiply the result by 2:
8 cm × 2 = 16 cm
The correct answer is 16 cm.
Supporting your child
Encourage your child to practise perimeter questions using different rectangles. Remind them to check they have included all sides, and to use the correct units in their answer.
Key insight: In GL and CEM style questions, shapes may be drawn in different orientations or include missing side labels. Encourage your child to label all sides and double-check their calculations before answering.
