How to find perimeter? Perimeter is a maths topic that goes hand-in-hand with area. If your child has managed to master the area topic, then good news, how to find perimeter is going to be a breeze! And if your child is completely confused by these maths topics, don’t worry! I will take you through everything they need to know step-by-step. In this blog post, I will go through the different types of perimeter questions that your child may come across throughout primary school and also in their transfer test preparation and revision.
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How to find the perimeter
The perimeter of a shape is the total distance around the edge or outside of the shape. Think of it as walking around the outside of a football pitch. That’s what perimeter is. We are adding up each of the edges. When your child first starts to learn about perimeter, they will be given basic 2D shapes such as squares and rectangles. However, as they progress throughout primary school, they will be given more complicated shapes with missing lengths and irregular shapes.
What is the difference between perimeter and area?
Like I said at the beginning of this blog post, perimeter and area and two maths topics that usually appear side-by-side, like Batman and Robin! This is why students can become confused between the two. If your child asks how to find the perimeter and area, keep reminding your child that the perimeter is the outside of a shape and the area is the inside of the shape. With perimeter, we add whereas with area we multiply.
How to find the perimeter of a shape
To calculate the perimeter of a shape, we add up all the sides of the shape. If the shape is a square or rectangle, this is nice and easy! However, if lengths are missing or the shape is an irregular shape, the adding up of its sides can become a bit more fussy. The unit of measurement with perimeter is just the same unit that is given in the question. There is no squared or cubed symbol after it. For example, if the perimeter is in centimetres then the unit of measurement stays the same, cm. Sometimes a question will ask you to give the perimeter in a different unit. For example, the question is in centimetres but students are asked to give their answer in metres. This will involve some metric conversion! (Don’t worry I’ll have a blog post on that too!)
How to find perimeter of triangle
Your child is probably familiar with finding the area of a triangle and if they haven’t come across that yet, don’t worry, they will soon enough! A maths question that isn’t as common BUT can still pop up in your child’s maths homework, is finding the perimeter of a triangle. And just like with any perimeter question, we add up all the edges of the triangle. As a triangle has three sides, we are adding up the three measurements which will be the two sides and the base of the triangle.
How to find perimeter of compound shapes
Compound shapes are shapes that are made up of 2D shapes. These shapes can look confusing and daunting when you first set eyes on them, but I promise, they aren’t that bad. There are a couple of steps needed to complete these types of perimeter questions but breaking it up into steps makes the process a lot simpler. Let’s go through an example together.
Step 1: Check we have all the measurements
The first thing that your child is going to do is check that the question has given them all the measurements that they need to find the perimeter of the shape. Sometimes with these types of perimeter questions, some of the sides are missing their measurements. If this happens, we are going to use the information that the question has given us to find the missing measurements. For example, with our question we can use the length of the shape on the left, to calculate the missing lengths on the right.
Step 2: Add up all the sides
The next step once we have all the measurements in place, is to add up all the edges. With compound area questions, this usually means adding up more than four numbers. It can be easy to get jumbled if there are a lot of numbers to add. So encourage your child to do their working out on the paper and add the numbers in batches rather than all at once. And voila! They have found the perimeter of a compound shape in two easy steps!
How to find the missing perimeter
So this is when our perimeter questions start to get a bit more complicated. Finding the missing lengths of a shape when the perimeter is given. A question where your child is most likely to do this is when they are given two shapes with the same perimeter but one of the shapes has missing measurements and also with compound perimeter questions which we already discussed above. Let’s take a look at an example of both types of questions.
Finding missing lengths in compound shapes:
We’ve already touched on this but I am going to go through another example. To find the missing lengths in the compound shape, we need to use the information that we have been given in the question. So we have an ‘E’ shape and some of the measurements are missing. We are going to use the measurements from the ‘E’ shape to find the missing lengths. Think of these measurements as little clues. If we look at the left side of the ‘E’ shape, we can see that we have the complete length of the shape, 20cm. If we look to the right of the shape, we can see we are missing three of the lengths but are given two lengths. We are going to use these two lengths on the right to find the three missing lengths.
The two lengths we have been given in the question are 4cm and 4cm. If we add these two lengths together we get 8cm. Next, we subtract 8cm from 20cm which leaves us with 12cm. Lastly, we will divide the 12cm between the three missing sides as we can see they are all going to be the same length. 12cm divided by 3cm gives us 4cm. So the missing lengths of the ‘E’ shape are 3cm each. One more thing to do and that’s add up all the sides! This gives us an answer of 100cm.
Using perimeter to find missing measurements
With this type of question, students are given two shapes which have the same perimeter but one of the shapes has missing measurements. For example, Shape A, a square, has a perimeter of 32cm. Shape B also has the same perimeter but it has missing measurements. We can see that one of the sides of Shape B, a rectangle, is 6cm. This means two of the sides add up to 12cm. If we subtract 12cm from 32cm this leaves us with 20cm. We will then divide 20cm by the two sides and that gives us 10cm. The missing measurement is 10cm.
Finding the perimeter when only the area is given
How to find the perimeter if you know the area? This type of perimeter question can make your mind work a bit harder! And that’s finding the perimeter when the area is given. With this type of question, we need to use the information that we have been given in the question. For example, if we have a rectangle which has an area of 64cm2 and one of the sides of the rectangle is 4cm, we need to divide 64cm by 4cm to find the length of the rectangle. When we divide 64 by 4, we get 16cm. This means the longer sides of the rectangle are 16cm. Now we just need to add up all of the sides of the rectangle. The perimeter of the rectangle is 40cm.
Perimeter Word Problems
It is an absolute certainty that your child will get some perimeter word problems coming up in their Key Stage 2 (KS2) maths homework. I’ve said this before and I will say it again, word problems are just a mathematical calculation with a story around them. Some perimeter word problems will include a graphic or drawing of a shape and some will just have the word problem. If it helps your child to complete the word problem, they can sketch out a little graphic or shape to help them with their working out.
Perimeter Transfer Test Questions
To be honest, I would rather a perimeter question come up in the transfer test rather than an area question. But like I said these two topics come hand in hand and chances are your child will get both types of questions coming up in the transfer test and practice papers. The types of perimeter questions that I have seen coming up in the transfer tests are:
- Perimeter of 2D Shapes
- Perimeter of triangles
- Perimeter of compound shapes
- Perimeter of shapes with missing lengths
- Perimeter word problems
Let’s go through an example of each of these types of transfer test perimeter questions and I will show you how to find perimeter in a variety of different questions.
Perimeter of 2D Shapes
The first and easiest type of perimeter question that can appear in the transfer test is finding the perimeter of a 2D shape. To find the perimeter of a shape, we add the edges of the shape together. Let’s look at an example of how to find perimeter rectangle?
Perimeter of triangles
This type of question is not as common but it has come up in practice papers before and that is finding the perimeter of a triangle. To find the perimeter of a triangle we add up the three edges. There are a few different types of triangles. We have right angle, equilateral, isosceles and scalene. Each type of triangle has different properties and this means the measurements will be different for each of these triangles. However, the method of finding the perimeter is the same, adding up all three edges. Here is an example of how to find perimeter of isosceles triangle.
Perimeter of compound shapes
The first step to finding the perimeter of a compound shape is to make sure that there are measurements for all of the edges. If measurements are missing, we need to use the information from the question to calculate the missing measurements before finding the perimeter of the shape. For example, Owen designed a games room for his children. Below is a plan of the games room. What is the perimeter of the games room? This question is an example of how to find perimeter of a compound shape.
Perimeter of shapes with missing lengths
Finding the perimeter of a shape with missing lengths is a perimeter question that I have seen popping up in the transfer test and practice papers. Remember to use the information from the question to find the missing lengths. For example, Shape A and Shape B have the same perimeter. What is the missing length from Shape B?
Perimeter word problems
If word problems make your child freak out and panic a bit, remind them that they’re just a mathematical calculation with a story around them. How to find perimeter in a word problem format? Let’s check out an example together. Monica is getting a new shed for her garden. The shed is rectangular in shape which measures 22m by 12m. What is the perimeter of the garden shed? With this question, we are finding the perimeter of a rectangle, there’s just a ‘story’ around this sum.
Another popular type of perimeter word problem that can come up in the transfer test is similar to this example. Below is a plan for a garden with a pond. The shaded area is the pond. What is the perimeter of the garden? With this question, students need to find the perimeter of the garden area minus the pond. The first step is to make sure all of the measurements are included. Nex,t we need to add up all of the edges of the garden. This will tell us the perimeter of the garden area.
More information on how to find perimeter
Teach your child how to find the area of shapes and make homework time easy!
I hope this has helped you understand all about perimeter and you can help your child smash their perimeter homework! Don’t forget to save this blog post incase you need a little reminder during homework time or transfer test preparation. If your child even gets confused and wonders how to find perimeter, come right back here!
