In this explainer, we will learn how to find the perimeter of a rectangle or a square by adding the lengths of all their sides or by formula using whole numbers only.
You should already know the definition of the perimeter of a shape.
The perimeter of a shape is the total length of its boundary. It is calculated by adding together the length of all the sides.
For a square with side length , the perimeter, which we can denote , will be equal to the sum of the lengths of its sides:
Additionally, since all the sides are equal in length, the perimeter be calculated by multiplying the length of one of the sides by 4:
This gives us a formula which describes how to find the perimeter of a square given its side length.
The Formula for the Perimeter of a Square
The perimeter of a square with side length can be calculated with the following formula:
Now that we know the formula for the perimeter of a square, we will use it to solve some typical problems.
Example 1: Finding the Perimeter of a Square given Its Length
Find the perimeter of a square whose side length is 19 cm.
We are told that the length of each side of the square has length 19 cm.
So, to find the perimeter, , we just have to add together the lengths of the 4 sides,
Alternatively, we can use the formula for the perimeter of a square which tells us that the perimeter is equal to where is the length of one of the sides. Hence,
But, this is not the final answer. We have to make sure we include the correct length units when recording the perimeter of shapes. Since the length of the side was measured in centimetres, the perimeter we have calculated is also in centimetres. So, the perimeter of the square is 76 cm.
Example 2: Finding the Perimeter of a Square and Converting Units
Calculate, in cm, the perimeter of a square with side length 70 mm.
We know that the perimeter of a square can be calculated with the formula , where is the length of one side. We know that and so
Since the length was given in millimetres, we have shown that the perimeter of the square is 280 mm.
Notice that the question asks us to calculate the length in centimetres. So, the final step is to convert 280 mm to centimetres. Recall that there are 10 mm in 1 cm, so to convert 280 millimetres into centimetres we have to divide by ten.
Hence, the perimeter of the square is
The above examples asked us to find the perimeter of a square given its length; a related question is to work backwards and find the side length of a square given its perimeter.
Since we know that , we can rearrange this formula by dividing both sides by 4 to obtain a formula to calculate the length given the perimeter .
Formula Linking the Length and Perimeter of a Square
The side length of a square with perimeter can be calculated with the following formula:
Example 3: Finding the Length of a Square given Its Perimeter
Calculate the side length of a square in metres whose perimeter is 24 m.
The perimeter of the square is 24 m, and we have been asked to find the side length .
The formulae linking the length and the perimeter of a square are
So, we calculate that
Finally, we must check that we use the correct units for our answer. The perimeter was given in metres and we were asked to find the length in metres so we conclude that the length is 6 m.
Example 4: Finding the Length of a Square given Its Perimeter
If a wire having a length of 96 cm is folded to form a square, how long will each side be?
The length of the wire is 96 cm and this is equal to the perimeter of the square that is created.
Since the 4 sides of the square have the same length, each side will be one-quarter of the length of the wire.
So, to find the length of a side we calculate
Finally, we must include the correct units in the final answer. Since the wire was measured in centimetres, we have shown that the length of each side is 24 cm.
Example 5: Solving Problems about Perimeter of Squares
The sum of the perimeters of two squares is 108 mm. If the side length of one of them is 2 mm, find the side length of the other square in millimetres.
It can be helpful to sketch what we know to help us see how to answer the question. We know that there are two squares; let’s call them A and B.
One of the squares has side lengths equal to 2 mm; we will say that that is Square A. So, the question is asking us to find the side length of Square B. We will denote the unknown side length by .
The last piece of information we are given is the sum of the perimeters, and we know that the perimeter of a square is equal to 4 times the side length.
Let’s write a formula for with this information.
Since the perimeter of Square A is equal to 8 mm and the perimeter of Square B is equal to , we have the following equation for the sum of the perimeters:
We can solve this equation to find .
First, subtract 8 from both sides:
Now, divide both sides by 4 to obtain
Finally, remember to include the correct units in your answer. The side length of Square B is 25 mm.
Finally, we will use what we know about the perimeter of squares to solve problems with more steps.
Here are the key steps to remember when solving problems with length and perimeter.
How To: Solving Questions Involving Length and Perimeter of Squares
- If you know the side length , you can multiply by 4 to obtain the perimeter .
- If you know the perimeter, you can divide by 4 to obtain the side length.
- Remember to always include length units in your answer, and check whether the question wants you to use the same units or convert between units.