Finding the perimeter of L-shapes and other compound rectilinear shapes by identifying and adding all outer edges.
Where your child meets this in real life: Measuring the boundary of an L-shaped room or calculating edging for an irregular garden
SEAGReady breaks perimeter of compound shapes into 2 steps, taught in order so each skill builds on the last.
Find the perimeter of an L-shape or compound shape when all outer edge lengths are labelled
Find the perimeter when some edge lengths must be calculated from given dimensions
Three free sample questions from our perimeter of compound shapes course. Every question comes with a full explanation, and hints that guide without giving the answer away.
Sean's bedroom is an L-shape. He wants to put LED lights around the edge of the room. All six walls are labelled: 6m, 2m, 4m, 3m, 2m, 5m. What is the total length of LED lights he needs?
Answer: A. 22 m
The perimeter is the total of all outer edges. Add all 6 sides: 6 + 2 + 4 + 3 + 2 + 5 = 22 m Sean needs 22 metres of LED lights.
Stuck? Start here: The perimeter is the total distance around the OUTSIDE of the shape.
Emma's garden is L-shaped. The total length is 10m and one section is 6m long. The total width is 8m and one section is 3m wide. Find the perimeter of the garden.
Answer: A. 36 m
Step 1: Find missing dimensions Missing length: 10 - 6 = 4 m Missing width: 8 - 3 = 5 m Step 2: Add all 6 sides 10 + 8 + 6 + 5 + 4 + 3 = 36 m The perimeter is 36 m.
Stuck? Start here: First find the missing lengths: if the total is 10m and one part is 6m, what is the other part?
An L-shaped playground has 6 sides with these measurements: 10m, 4m, 6m, 5m, 4m, 9m. What is the perimeter of the playground?
Answer: A. 38 m
Add all 6 outer edges: 10 + 4 + 6 + 5 + 4 + 9 = 38 m The perimeter of the playground is 38 m.
Stuck? Start here: Perimeter means adding ALL the outer edges together.
This is the exact interactive worked example your child sees in SEAGReady. Step through it and watch the method build up.
Ciara's bedroom is an L-shape. She wants to put fairy lights around the edge of her room. All six walls are labelled: 8m, 3m, 5m, 4m, 3m, 7m.
What is the total length of fairy lights she needs?
8 + 3 + 5 + 4 + 3 + 7
Step 1 of 4
Ciara's bedroom is an L-shape. She wants to put fairy lights around the edge of her room. All six walls are labelled: 8m, 3m, 5m, 4m, 3m, 7m.
What is the total length of fairy lights she needs?
Ciara needs 30 metres of fairy lights.
The key insight: Perimeter means the OUTER boundary only - go all the way around the outside!
Watch out: Including internal lines in the count. Internal lines are not part of the perimeter - only count edges on the outside.
These are the misconceptions we see most often in perimeter of compound shapes, including the ones our practice questions are specifically designed to catch.
Struggling with perimeter of compound shapes? The real gap is often in one of these earlier topics.
SEAGReady finds the exact step where your child gets stuck, teaches it with worked examples like the one above, and brings it back for review so it sticks.