The word is long, and that is a handy way to remember that perimeter refers to the length, or distance, around the outside of a 2D shape, or polygon. By adding the length of each side in our shape, we find out the total length, which is the perimeter.

Let's see how we do this with some shapes, and also how the unit of measurement is really important here.

Worked Example

Question 1

Find the perimeter of the shape given.

Looking for clues

What if there were some shapes that had special features that made it easier to work out the perimeter? Well, there are! Lots of them, in fact. Let's take a look at how we can calculate the perimeter of a shape, when some sides don't have the length written on them!

Remember!

Even if we think we don't have enough information to work out the perimeter, the shape may have some special features that do make it possible. Look for sides of equal length to see if you have enough information to calculate the perimeter.

Worked Example

QUESTION 2

A rectangle has a perimeter of $22$22cm. If its length is $8$8cm, what is its width?

Now what?

We've worked out how to calculate the perimeter of a shape and even found some shortcuts. What happens if we already know the perimeter, but need the length of some sides? Here's what we can do:

Remember!

If the perimeter is the total of the length of each side of our shape, then we can work in the other direction to find out the missing length of a side.

Worked Example

Question 3

Find the side length $d$d indicated on the diagram. The perimeter of the shape is $22$22m.

Outcomes

9D.MG2.03

Solve problems involving the areas and perimeters of composite two-dimensional shapes