11+ Maths: Compound Shape Perimeter Practice (CEM & GL)
19 October 2026
Understanding compound shapes
Compound shapes are made by joining two or more simple shapes, such as rectangles or triangles. For 11+ exams, children are often asked to find the perimeter. The total distance around the outside edge. It's important to spot which lines form the boundary and which are inside the shape.
Finding the outer edges
Start by tracing the outline of the shape with your finger or a pencil. Only the outside edges count towards the perimeter. Internal lines, added for splitting the shape, should be ignored when adding up the total length.
Calculating missing lengths
Sometimes, not all side lengths are given. Encourage your child to look for clues: often, the total length of one side can be worked out by adding or subtracting other labelled lengths. Drawing extra lines or labelling each edge can help keep track.
Adding up the perimeter
Once all the outside edges are known, add them together carefully. Remind your child to include each segment only once, and to double-check their list of sides before calculating the final answer.
Key insight: Internal lines are a common trap
It's easy for children to accidentally include lines that are inside the shape, especially when the compound shape is split into rectangles. Double-checking which lines are truly on the outside can prevent simple mistakes and boost confidence.
