Understand the idea.
Walk around once
To measure perimeter, start at one corner and follow the outside boundary until you return to that corner. Read or measure every side you pass. Add each boundary length once, marking sides as you count them.
Include short sides as well as long ones. A dividing line drawn inside the shape is not part of this outside walk, so it does not contribute to the perimeter.
Use matching units
Use the same length unit for every side before adding. On a rectangle, opposite sides are equal, so two labelled neighbouring sides tell you all four lengths. For a 7 cm by 4 cm rectangle, use 7, 4, 7 and 4 cm.
Perimeter remains a length, so report centimetres or metres, not square units. A quick check is to add a neighbouring pair and double it for a rectangle; this includes both pairs of opposite sides.
See the method in action.
Rectangle boundary
Find the perimeter of the 7 cm by 4 cm rectangle.
Worked solution
- 1
List the four boundary lengths.
Opposite sides of a rectangle are equal, even where a label is only printed once.
- 2
Add the sides around the boundary.
The sum includes all four outside sides once.
- 3
Check using two equal pairs.
The rectangle has two copies of the neighbouring 7-centimetre and 4-centimetre sides.
22 cm.
An irregular fence
A four-sided garden has sides 5 m, 4 m, 6 m and 3 m. What length of fence surrounds it?
Worked solution
- 1
Identify every fence section.
All four given lengths form the outside boundary of the garden.
- 2
Add convenient pairs.
The fence must cover the complete boundary, so every listed side contributes.
- 3
Report the required fence length.
Metres are the given length unit; no area covering is being measured.
18 m.
Try it yourself.
Check your understanding
What is the perimeter of a square with sides 5 cm?
Show answer & reasoning
20 cm.
A square has four equal sides. Add four 5-centimetre lengths: , or .
Keep practising
6 worksheet questions · 8 marks
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
cm.
12 cm. - Find the perimeter of a rectangle 8 cm long and 2 cm wide.1 mark
cm.
20 cm. - A triangular frame has sides 4 cm, 5 cm and 6 cm. Find its perimeter.1 mark
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
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.
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 ↗