11+ Maths: Compound Shape Perimeter Practice (CEM & GL)
26 October 2026
Understanding compound shapes
A compound shape is a 2D figure made by joining two or more rectangles together. In 11+ exams, these shapes often look like L-shapes or rectangles with parts missing. Children are asked to find the perimeter, which means the total distance around the outside of the shape.
Why perimeter of compound shapes matters
Perimeter questions test more than just adding numbers. They check if children can spot missing side lengths and keep their working organised. CEM and GL exams often leave out some lengths on purpose, so finding these is part of the challenge.
Tips for practising perimeter questions
Encourage your child to trace the outline of the shape with a finger or pencil, noting each side as they go. Remind them to look for sides that are not labelled and work out their lengths by comparing other sides. Check that every edge around the shape has been counted once.
Worked example
Let's find the perimeter of this L-shaped compound shape:
The shape has these labelled sides:
Top: 8 cm
Right: 4 cm
Bottom right: 3 cm
Bottom: 8 cm
Left: 7 cm
Left inner: 3 cm
Step 1: Identify any missing sides. The inner horizontal side on the left is not labelled.
Step 2: Work out the missing side. The total height on the left is 7 cm. The two vertical sections on the right add up to 4 cm + 3 cm = 7 cm, so the shape's height is correct. The two horizontal sections on the bottom add up to 8 cm (bottom) and 3 cm (bottom right), which is 11 cm. The top is 8 cm, so the missing inner horizontal side is 3 cm (11 cm - 8 cm = 3 cm).
Step 3: Add all the sides:
8 cm (top) + 4 cm (right) + 3 cm (bottom right) + 8 cm (bottom) + 3 cm (left inner) + 7 cm (left)
Step 4: Calculate the total:
8 + 4 = 12
12 + 3 = 15
15 + 8 = 23
23 + 3 = 26
26 + 7 = 33
The correct perimeter is 33 cm.
Common pitfalls and how to avoid them
Children often miss out a side, add a side twice, or confuse area with perimeter. Remind your child to only add the sides around the edge, and to double-check that each side is included once.
Key insight: Perimeter questions with compound shapes are less about complex maths and more about careful, organised working. Practising the walk-around method helps children avoid simple mistakes and gain confidence.
Summary
Understanding compound shapes and practising the perimeter method step by step will help your child approach these questions with confidence. Encourage them to trace every edge, find missing lengths, and add each side only once for an accurate answer.
