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Calculating the perimeter and area of irregular shapes

Calculating the perimeter and area of irregular shapes
A teacher-created KS2 worksheet on calculating the perimeter and area of irregular shapes to support your child's maths learning at home.

Once you know how to find the area of a rectangle, you can use this knowledge to find the area of irregular, compound shapes as long as they are made up of rectangles or squares. Can you calculate the area and perimeter of the shapes below?


Keystage:  KS2, Year 5

How do you find the perimeter of an irregular shape in KS2 maths?

To find the perimeter of an irregular shape in KS2 maths, you simply add up the lengths of all the sides of the shape.

Identify the sides: Look at the shape and find the length of each side. Sometimes, these lengths will be given, but other times, they might need to be measured.

Add the lengths together: Once you know the length of each side, add them all up. The total you get is the perimeter.

Here's an example: 

If an irregular shape has four sides with lengths of 3 cm, 5 cm, 4 cm, and 6 cm, you would add those numbers together:

3 cm + 5 cm + 4 cm + 6 cm = 18 cm.

So, the perimeter of the shape is 18 cm.

That’s all there is to it! Just add up the lengths of all the sides, and you’ve got the perimeter.

How do you find the area of an irregular shape in KS2 maths?

Finding the area of an irregular shape in KS2 maths can be a bit trickier, but here's a simple way to explain it:

Break the shape into smaller regular shapes

  • Look at the irregular shape and see if you can divide it into smaller shapes that are easier to work with, like rectangles, squares, triangles, or other shapes whose area you know how to find.

Find the area of each smaller shape

  • Calculate the area of each of these smaller shapes. For example, for a rectangle, you multiply the length by the width. If it's a triangle, you might use the formula (base × height) ÷ 2.

Add the areas together

  • Once you've found the area of each smaller shape, simply add them all together to get the total area of the irregular shape.

Here's an example:

Imagine you have an L-shaped figure. You could break it into two rectangles. If one rectangle is 4 cm by 3 cm and the other is 2 cm by 3 cm, you'd find the area of each:

  • First rectangle: 4 cm × 3 cm = 12 cm²
  • Second rectangle: 2 cm × 3 cm = 6 cm²

Then, add these together:

12 cm² + 6 cm² = 18 cm²

So, the area of the L-shaped figure is 18 cm².

That’s how you find the area of an irregular shape in KS2 maths—by breaking it down into simpler parts, calculating each area and then adding them up.

For more help with KS2 maths, visit our Maths hub page, or try a new challenge such as our Types of angles worksheet.