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KS3 Bitesize



The perimeter of a shape is the distance around its edge. Finding the perimeter just means adding up the lengths of all its sides.


This Revision Bite covers:

Simple shapes

The perimeter of a shape is the length of its boundary.

Think of an ant starting from one corner of a small box, measuring 3 cm by 4cm. Imagine it walks all the way around the edge of the box. What distance will it have walked?

A box with 2 sides measuring 4cm wide and 2 sides measuring 3cm tall.

Perimeter = 4 + 3 + 4 + 3 = 14 cm

When calculating the perimeter, remember to include the length of every side.

Compound shapes

Sometimes the shape is more complicated. In this case you need to calculate 'missing' lengths and be particularly careful to include all the sides.

An L shape, with two sides measuring 2 cm, two sides measuring 3 cm, and two sides measuring 5 cm.

Perimeter = 2 + 2 + 3 + 3 + 5 + 5 = 20 cm


A plan of a play area is shown below:

Perimeter = 15 + 20 + 8 + 5 + x + y

a) Calculate the length of x and y
b) Calculate the perimeter of the play area.


a) The length of the play area is 20m, so x = 20 - 8 =12m.

The width of the play area is 15m, so y = 15 - 5 = 10m.

b) Perimeter = 8 + 5 + 12 + 10 + 20 + 15 = 70m.

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