Perimeter
The perimeter is the total distance around the edge of a 2D shape. Examples of metric 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?
Image caption, To calculate the perimeter, label the length of all the sides
1 of 2
The perimeter of any 2D shape can be found by labelling and adding together the lengths of the sides.
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?
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
This shape is made from two rectangles.
What is the perimeter of this shape?
First, calculate the missing lengths \(x\) and \(y\).
\(x = 20 - 8 = 12~\text{m}\)
\(y = 15 - 5 = 10~\text{m}\)
Second, label each length.
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 added.