Perimeter

The perimeter is the total distance around the edge of a 2D shape. Examples of units of distance are millimetres (mm), centimetres (cm) and metres (m).

Example

A fence is built around the perimeter of a field. What is the total length of the fence?

Two sides labelled 5 & 10 cm

To calculate the perimeter, label the length of all the sides

The perimeter of any 2D shape can be found by labelling and adding together each side.

Remember to include the length of each side when calculating the perimeter.

Question

What is the perimeter of an equilateral triangle, when the length of one side is 4 cm?

Equilateral triangle with each side measuring 4cm

An equilateral triangle has three sides of equal length. The perimeter of the equilateral triangle is 4 + 4 + 4 = 12 cm

Perimeter of a compound shape

A compound shape is made up of several simple 2D shapes. To calculate the perimeter, label and add up the length of each side.

Example

What is the perimeter of this shape?

6 sided shape measuring 8 x 5 x x x y x 20 x 15m

First, calculate the missing lengths x and y.

x = 20 - 8 = 12~\text{m}

y = 15 - 5 = 10~\text{m}

Second, label each length.

6 sided shape measuring 8 x 5 x 12 x 10 x 20 x 15m

Add up all the sides to calculate the perimeter.

8 + 5 + 12 + 10 + 20 + 15 = 70~\text{m}

Remember to make sure that all the sides have been included.