Lesson video

- Hello, everybody.

My name is Mr. Kelso, and welcome to today's lesson on measuring and calculating perimeter of a shape.

Now, before we start, you'll need a pen or pencil and a piece of paper, as well as a ruler.

Also, please try and find a quiet place somewhere that you're not going to get disturbed.

And don't forget to remove any sorts of distractions.

For example, put your mobile phone on silent or move it away completely.

Pause the video and then when you're ready, let's begin.

Today's lesson is about calculate and measure the perimeter of a shape.

We'll start by calculating the perimeter in simple shapes.

Then we'll look at calculating the perimeter in compound shapes.

We'll take on our independent task, and then it's time for a quiz.

You'll need a pencil, ruler and a piece of paper.

So our warmup activity.

Practise measuring some things in the house such as a book, a light switch, or a picture frame.

When measuring, remember to line up the end of the object with zero, not the end of the ruler.

Also look accurately at where it comes on the ruler.

In the example opposite, the pencil is seven centimetres and it's one millimetre.

We could also write this as 7.

1 centimetres, or we could write it as 71 millimetres.

Pause the video, have a go at that task.

In order to access this lesson, we first need to consider what does perimeter mean? Take a moment and think if you can come up with a definition of a perimeter.

Think if you can find some examples around your room.

So the perimeter is the distance around the boundary of a shape or around the outside of a shape.

Have a look at this shape.

It's a rectangle.

See if you can find the perimeter of this shape.

These units will help so you don't have to count individual squares.

So we are looking to measuring the outside of the shape.

So 11 units and four units is 15.

11 and four is another 15.

So the perimeter is 30 units.

You'll notice that I counted 11, four, 11, four.

In a rectangle, you could do this a little bit differently.

You count 11 units and 11 units, and then four units and four units.

Alternatively, you might want to do four units and 11 units and then multiply it by two to get your other four units and 11 units.

Lots of different ways to find the perimeter, but the important part is you're finding the distance around the boundary of a shape.

Now it's your turn.

Calculate the perimeter of the shapes below.

Look carefully at the units and remember, these are not drawn to scale.

The first shape is a square.

Pause the video, and when you're ready, press play to continue.

You'll notice that the first shape is a square.

That means that all the sides are equal length.

So four lots of 23 add up to 92 centimetres.

Next shape is a rectangle, so I'm gonna add my 12 millimetres at the bottom and the top together.

I'm then gonna add my 32 millimetres at the sides together to get 88 millimetres.

My final shape is an isosceles triangle, so I'm gonna add eight metres and eight metres and two metres to get 18 metres in total.

- Great stuff, Mr. Kelso.

Hi, everyone.

My name's Ms. Jones, and I've got one more example for you.

Have a look at this shape.

What do you notice about it? Look carefully at the units of measure.

That's right, they're using mixed units.

So we can see our length is measured in centimetres and our width is measured in millimetres.

So what will I need to do before I calculate the perimeter here? Well, I'm gonna go through two options with you and hopefully, you can apply this to some questions later.

Option A, we can convert both of these to centimetres, okay? Because we need them to be in the same unit when we're adding them all up so our answer is also in that same unit.

So I convert both to centimetres.

I know that 24 millimetres is equal to 2.

4 centimetres because there are 10 millimetres in every centimetre.

So then if I add up each side, I get a total of 21.

4 centimetres.

What else could I have done instead? What could I have done differently? Option B, I could have converted both to millimetres and then added them up.

So I would think about the one that's in centimetres, 8.

3, and I know that 8.

3 centimetres is equal to 83 millimetres.

Then again, I add my sides up and I get 214 millimetres.

Now, both of these answers are correct and equivalent, but we need to make sure that we've got the same unit of measure when we're doing our calculations.

Okay, I'm gonna hand you back over to Mr. Kelso and then he's going to explain some questions that you can have a go at.

- Your turn.

Choose your strategy to find the perimeter of the shapes below.

Pause the video and when you're ready, press play to continue.

So the answers are on the screen.

On the first rectangle, the answer is either 26 centimetres and six millimetres.

You could write this as 26.

6 centimetres, or you could write this as 266 millimetres.

The second shape, you could write this as 262 millimetres, 26.

2 centimetres, or 26 centimetres and two millimetres.

All three of the answers are correct.

How would I calculate the perimeter of this compound rectilinear shape? Think about starting in the corner and working all the way around the shape until you come back to the corner.

Pause the video, have a go.

When you're ready, press play.

So we just need to add up all of the side lengths.

We can start in one corner.

We can add 12, add four, add two, add seven, add six, add seven, add four, add four, and it gives us a total of 46 centimetres.

The shape is a little bit different.

What do you notice about the measurements on this compound shape? Not all of the measurements are given, so we need to do a little bit of detective work.

I need to find out what is the length of this side here.

Well, I know the bottom length is eight centimetres and the top length is four centimetres, so that means that it must be another four centimetres.

Side length is a total of six centimetres, and I've used four on the left-hand side, which means there must be two centimetres there.

Once I've got this information, I can then find the perimeter of this shape.

Add all of the side lengths to find the perimeter.

So add four, add six, add eight, add four, add four, add two gives a total of 28 centimetres.

Pause the video and now it's time to complete your task.

Press play when you're ready.

Question one asks to calculate the perimeter of the shapes below.

You see the first shape is 62 centimetres.

The square is 176 centimetres.

The hexagon is 60 centimetres.

The isosceles triangle is 29 millimetres, and the pentagon, the regular pentagon is 10 centimetres.

Question number two, calculate the perimeter of these shapes.

Pause the video and when you're ready, press play to continue, I know that I'm converting to different units, so I can convert between centimetres to millimetres or millimetres to centimetres.

The perimeter of the rectangle is 72.

6 centimetres or 726 millimetres.

The hexagon and the isosceles triangle is 212 millimetres or 21.

2 centimetres.

Question three asks to calculate the perimeter of the compound shape.

Pause the video.

When you're ready, press play to continue.

Count around the outside, one, add five, add four, add two, add one, add one, add two, add four, and I'm back to the beginning.

It adds up to a total of 20 centimetres.

Question four, calculate the perimeter of this compound shape.

Think about this one.

Are there any missing sides? Pause the video and when you're ready, press play to continue.

I can see that I have a missing side, and I know that two, add three, add one centimetre, the three values here add up to my six centimetres here.

Once I've found this missing side, I can then go back to calculating the perimeter.

Five and six and 10, one, six, three, three, two, which adds up to 36 centimetres.

Congratulations on completing your task.

If you'd like to, please ask your parent or carer to share your work on Twitter, tagging @OakNational and also #LearnwithOak.

Now, before we go, please complete the quiz.

So that brings us to the end of today's learning on measuring and calculating the perimeter of a shape, a really big well done on all the fantastic learning that you've achieved.

Now, before you finish, perhaps quickly look back at your notes from today and try and identify the most important parts of your learning.

Well, all that's left for me to do is say thank you, take care, and enjoy the rest of your learning for today.