GEOMETRY

Perimeter

Learn the reliable method for perimeter, then apply it to 11+ exam-style questions.

1 · WHAT YOU NEED TO KNOW

The idea

Perimeter is the total distance around the outside boundary.

Picture itTrace the distance around the edge
Colourful playground markings showing shapes, equal sections, parallel lines and boundaries
2 · THE METHOD

Follow these steps

  1. Mark every outside edge once.
  2. Find missing side lengths from matching totals where needed.
  3. Add the boundary lengths and include a linear unit.
perimeter = total distance around the outside
3 · WORKED EXAMPLES

See the method in action

Example 1

The diagram shows two congruent rectangles, each measuring 18 cm by 13 cm. They are joined along an edge of length 13 cm. What is the perimeter of the combined shape?

18 cm13 cmshared edge 13 cmnot drawn to scale
  1. The two separate perimeters total 124 cm.
  2. The shared 13 cm edge is internal and was counted twice, so subtract 26: 124 − 26 = 98 cm.
Answer: 98 cm
Example 2

Two display boards have the dimensions shown. How much more edging is needed for the board with the greater perimeter?

A: 16 cm × 12 cm
B: 14 cm × 13 cm
  1. Rectangle A needs 56 cm of edging and B needs 54 cm.
  2. The difference is 56 − 54 = 2 cm.
Answer: 2 cm
4 · ELEVENSPRINT TIP

A useful shortcut

Add the boundary lengths and include a linear unit.

5 · COMMON MISTAKES

Watch out for these

6 · TRY IT YOURSELF

Three fresh questions

Ready to check the method? You will get three fresh, validated questions.