01Why this matters
Fencing, garden edging and picture frames require boundary length. This is different from how much surface a shape covers.
02See it
03Understand it
Perimeter is the total distance around the outside boundary of a flat shape.
Add every outside side once. For a rectangle, opposite sides are equal, so P = 2l + 2w. Convert all lengths to the same unit before adding. Interior lines do not count unless they are part of the boundary being measured.
04Guided example
Find the perimeter of a rectangular garden 8 m long and 3 m wide.
- The four boundary lengths are 8 m, 3 m, 8 m and 3 m.
- Add them: 8 + 3 + 8 + 3 = 22.
- The perimeter is 22 m. It is a length, so the unit is metres, not square metres.
05Try it
06Practise it
07Use it in the real world
A 1 m gate replaces part of a 22 m garden boundary. The remaining fence length is 21 m before allowing for posts, joins and waste.
Go a little deeper
Shapes can have the same perimeter but different areas. A 1 m by 9 m rectangle and a 5 m by 5 m square both have perimeter 20 m, but areas 9 m² and 25 m².
08Quick check
09Summary
Perimeter is the total distance around the outside boundary of a flat shape. The perimeter is 22 m. It is a length, so the unit is metres, not square metres.
Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.
10Key idea
P = 2(l + w)
P is perimeter; l is length; w is width, all in the same length unit.
w = P/2 − l
Find this in the formula library →