The idea, step by step
Walk around once
Imagine starting at one corner and walking all the way around the shape to return to your starting point. Measure or read each outside side and add those lengths. Mark a side after counting it so no side is missed or counted twice.
Use matching units
All lengths must use the same unit before they are added. For a rectangle, opposite sides have equal lengths. A 7 cm by 4 cm rectangle has sides 7, 4, 7 and 4 cm, giving a perimeter of 22 cm. A line drawn inside a shape is not part of its outside perimeter.
A common trap
Only adding the two labelled sides of a rectangle.
Instead: A rectangle has four sides. Use the opposite-side lengths as well.
See the thinking
Rectangle boundary
Find the perimeter of the 7 cm by 4 cm rectangle.
- Read all four sides: 7, 4, 7 and 4 cm.
- Add 7 + 4 + 7 + 4 = 22.
An irregular fence
A four-sided garden has sides 5 m, 4 m, 6 m and 3 m. What length of fence surrounds it?
- Every listed side is on the outside boundary.
- 5 + 4 + 6 + 3 = 18.
- Use metres because the side lengths are in metres.
Check your understanding
What is the perimeter of a square with sides 5 cm?
Show answer
20 cm.
5 + 5 + 5 + 5 = 20 cm.
Your turn
6 questions · 8 marks · Work at your own pace.
Read the questions online
- Find the perimeter of a square with sides 3 cm.1 mark
- Find the perimeter of a rectangle 8 cm long and 2 cm wide.1 mark
- A triangular frame has sides 4 cm, 5 cm and 6 cm. Find its perimeter.1 mark
- A five-sided shape has side lengths 2 m, 3 m, 4 m, 3 m and 2 m. Find its perimeter.1 mark
- A square garden has a perimeter of 24 m. Find one side length.2 marks
- A 9 cm by 4 cm rectangle has a line drawn across its middle. Does that line change its perimeter? Explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Find the perimeter of a square with sides 3 cm.1 mark
3 + 3 + 3 + 3 = 12 cm.
12 cm. - Find the perimeter of a rectangle 8 cm long and 2 cm wide.1 mark
8 + 2 + 8 + 2 = 20 cm.
20 cm. - A triangular frame has sides 4 cm, 5 cm and 6 cm. Find its perimeter.1 mark
4 + 5 + 6 = 15 cm.
15 cm. - A five-sided shape has side lengths 2 m, 3 m, 4 m, 3 m and 2 m. Find its perimeter.1 mark
2 + 3 + 4 + 3 + 2 = 14 m.
14 m. - A square garden has a perimeter of 24 m. Find one side length.2 marks
Four equal sides make 24 m.
24 divided by 4 is 6 m.
6 m. - A 9 cm by 4 cm rectangle has a line drawn across its middle. Does that line change its perimeter? Explain.2 marks
The inner line does not change the outside boundary.
9 + 4 + 9 + 4 = 26 cm.
No. The perimeter stays 26 cm.
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics 2026
Boundary/interior; measuring perimeter; covering/counting/estimating area; comparing shapes.
Official curriculum source ↗