What Is Perimeter? Explained For Elementary School

The perimeter of a shape is the measurement around its edge (the perimeter of a circle is called the circumference).

This blog is part of our series of blogs designed to explain key math concepts for those supporting elementary school children or as part of home learning. 

What is perimeter?

The perimeter of a math shape is the total measurement of all the edges of a shape e.g. a triangle has three edges, so its perimeter is the total of those three edges added together.

A square’s perimeter is easy to calculate if one side is given as all sides are the same length; the perimeter of a square with side length 5 cm is 20 cm, because 5 x 4 = 20. The perimeter of a rectangle can be calculated by adding the length and width together and doubling it.

For the perimeter of a rectangle with a length of 5 cm and width of 3 cm can be calculated as 5 + 3 + 5 + 3 (two lots of the length + two lots of the width), or double 5 + 3, which is 16 cm.

How to measure the perimeter of a shape

When measuring the perimeter of more complex shapes, children could be encouraged to highlight each edge as they add it together to ensure they don’t miss any out.

When will my child learn about perimeter in elementary school?

Perimeter is taught as children begin to learn geometry. Children begin by learning to measure the perimeter of simple 2-D shapes in 3rd grade. Children will also measure and calculate the perimeter of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter

In 4th grade, children apply the perimeter formula for rectangles in real world and mathematical problems. For example, find the width of a rectangular room given the area of the flooring and the length, by viewing the area formula as a multiplication equation with an unknown factor.

How does perimeter relate to other areas of math?

Perimeter is most closely linked to properties of shapes, 2D shapes and 3D shapes, and is one of the most basic and important parts of geometry.

A good understanding of how to calculate the perimeter of a shape is needed before children can begin to learn more complex geometric ideas such as area and volume.

Wondering about how to explain other key math vocabulary to your children? Check out our Math Dictionary for Kids, or try these math terms:

• What Is A Venn Diagram: Explained For Teachers, Parents and Kids
• What Is Place Value: Explained For Teachers, Parents And Kids
• What Is The Highest Common Factor: Explained For Teachers, Parents And Kids

Practice perimeter questions

1. Calculate the perimeter of this square.    

(Answer: 8 x 4 = 32cm)

2. Sam drew a rectangle with a perimeter of 28cm. His rectangle was 10cm long. How wide was it?

(Answer: 4cm)

3. Here are some shapes on a 1cm square grid. a) What is the perimeter of shape A? b) Which shape has the smallest perimeter?

(Answer: a) 14cm b) D)

4. Megan says, ‘If two rectangles have the same perimeter, they must have the same area.’ Is she correct? Explain how you know.

(Answer: No – 20cm perimeter could be 2 x 10 or 4 x 5)

5. Here is an equilateral triangle inside a square. The perimeter of the triangle is 48cm. What is the perimeter of the square?

(Answer: 64cm (48/3 = 16 (one side of the triangle) so 16 x 4 = 64)

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The content in this article was originally written by primary school teacher Sophie Bartlett and has since been revised and adapted for US schools by elementary math teacher Christi Kulesza.