Lesson 14 — Activity 2: Perimeter of Irregular Shapes

Getting Ready

You have spent time practising how to determine the perimeter of regular shapes. At times, you may be asked to find the perimeter of shapes that are not regular.

Irregular shapes are shapes that have sides of any length. You will encounter these kinds of shapes often in everyday life.

Rooms in homes often have irregular shapes.

Courtesy of Getty

Think about This:

Think about other shapes you may need to measure.

You may wish to measure for this kind of swimming pool.

Courtesy of Getty

You may be able to think of many more examples of irregular shapes in your everyday life!

Finding Perimeter of Irregular Shapes

To find the perimeter of an irregular shape with straight sides, you simply count the lengths around the entire object.

Courtesy of ADLC

For the shape above,you would do the following:

P = 1 cm + 4 cm + 2 cm + 5 cm + 6 cm = 18 cm

Try This:

Determine the perimeter for this shape.

Courtesy of ADLC

P = 4 m + 2 m + 3 m + 4 m + 3 m + 2 m = 18 m

You may be asked to find the perimeter of an irregular shape where one (or more) of the measurements of the sides are missing.

Watch this video that explains how to determine the length of the missing side(s).

As the video showed, you can use opposite sides to find missing values. Here's one more example, this time with two missing sides.

Courtesy of ADLC

To find the length of the left-hand side, add the opposite sides:

4 cm + 3 cm = 7 cm. Therefore, the left-hand side is 7 cm.

To find the length of the bottom side of the shape, first look at the top of the shape. It measures 6 cm. Then notice the small side opposite, it is 2 cm. To find the missing bottom side, you would do this:

6 cm – 2 cm = 4 cm. Therefore, the bottom side is 4 cm.

Now that you know the length of all the sides, you can calculate the perimeter.

P = 6 cm + 4 cm + 2 cm + 3 cm + 4 cm + 7 cm = 26 cm


Try This!

Look at the shape below. Determine the perimeter.


Courtesy of ADLC

P = 1 cm + 4 cm + 4 cm + 1 cm + 4 cm + 4 cm = 18 cm

Now look at this shape below. First, find the length of the missing sides, and then add up all the sides to find the shape's perimeter. You may use a calculator to solve.

Courtesy of ADLC

25 mm – 18 mm = 7 mm (for vertical side)
22 mm – 6 mm = 16 mm (for horizontal side)
P = 25 mm + 6 mm + 7 mm + 16 mm + 18 mm + 22 mm = 94 mm

Digging Deeper

Click here to review perimeter and also to learn about determining the perimeter of shapes with curved edges.

Watch the video from the beginning until the end of Topic 4.