Perimeter of Rectangles and Simple Shapes

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Draw the Rectangle in Your Copy
5 Class Challenge
6 What Did We Notice?
7 What's Next
Pupil practice

1 - Getting Started ~4 mins

Key point

Look at the rectangle on the board. Only two sides have numbers: the top is 6 cm and one side is 3 cm. The other two sides are blank.

Have a think: what must the bottom side be? And the other side? How could you be so sure without measuring them?

Teacher Notes

Take three hands-up answers, not open call-outs. Hold out for why pupils are sure of the two blank sides, not just what the numbers are.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

Watch four shapes on the board. For each rectangle, notice which two sides match, so we can fill in the sides that have no number before we add. The last one is an L-shape, so look carefully at how many sides it really has.

Shape Measurer

Teacher Notes

Work each shape live; the readouts are on because these are the worked examples.

6 by 3 rectangle — draw out that top matches bottom and one end matches the other. Add 6 + 3 + 6 + 3 aloud with the class.
5 by 5 square — a square is the special rectangle where all four sides match. Four fives.
7 by 2 rectangle — the long, thin one; the two twos are easy to forget.
L-shape — each of the six sides shows its length on screen. Count the sides together first (there are six), then add them one at a time, touching each only once.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~10 mins

How we work

A pupil takes a turn at the board while the rest of us work out the perimeter aloud first, then that pupil types in our answer. We fill in the sides we were not given, then add all four.

Our shapes:

4 cm by 3 cm rectangle
6 cm by 2 cm rectangle
5 cm by 5 cm square — all sides match
8 cm by 3 cm rectangle — don't miss the two threes
L-shape — count all six sides carefully before you add

Find the missing sides, then the perimeter

Teacher Notes

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud. The perimeter stays hidden until we type in our answer, so we must work it out first.

For each rectangle, get the two given sides read aloud, then ask the class for the two matching opposite sides before anyone adds.
Watch for pupils adding only the two numbers they were given (6 + 3 = 9) and forgetting the matching pair. Head this off by pointing at each of the four sides in turn as they add.
The square is the one to slow down on: draw out that every side matches here, so it is four of the same number.
The L-shape is new: count all six sides aloud as a class first, then add them one at a time so none is missed or counted twice.

+15 XP

4 - Draw the Rectangle in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, draw a 5 cm by 3 cm rectangle using your ruler. Write the length on all four sides, so both 5 cm sides and both 3 cm sides have a number. Then write the perimeter sentence underneath with its total, like this: 5 + 3 + 5 + 3 = ___ cm.

Teacher Notes

Walk the room glancing that all four sides carry a number, not just the two you drew first — this is whole-class copybook practice, not marking. Nudge anyone who only wrote two lengths to fill in the matching opposite pair.

+10 XP

5 - Class Challenge ~8 mins

Now we work these out on the board together, filling in any missing sides first. Draw each shape as we go so we can trace right around its edge.

Our shapes:

4 cm by 4 cm square
6 cm by 3 cm rectangle
7 cm by 4 cm rectangle
L-shape from two rectangles — count the six sides carefully

Example

For the L-shape, the six sides going around are 5 cm, 2 cm, 3 cm, 4 cm, 2 cm and 2 cm.

Work out the perimeter of each shape. Fill in any missing sides first, then add every side once. For the L-shape, use the labelled figure drawn on the board.

For each rectangle, fill in the two missing opposite sides, then add all four. For the L-shape, use the labelled board figure: count and check every side, then add them one at a time.

A 4 cm by 4 cm square flag. What is the distance all the way around its edge?
A 6 cm by 3 cm garden bed. What is its perimeter?
A 7 cm by 4 cm noticeboard. What is its perimeter?
The L-shape drawn on the board, with sides 5 cm, 2 cm, 3 cm, 4 cm, 2 cm and 2 cm going around. What is the distance all the way around?

Ways to start:

Read out the two sides you were given, then say what the two matching sides must be.
On the L-shape drawn on the board, trace one full trip around the edge with your finger, saying each side length as you touch it.

Stretch:

On the L-shape drawn on the board, how do you know you have counted every side and none of them twice?

Answers & strategies (teacher)

A 4 cm by 4 cm square flag. What is the distance all the way around its edge? — 16 cm
A 6 cm by 3 cm garden bed. What is its perimeter? — 18 cm
A 7 cm by 4 cm noticeboard. What is its perimeter? — 22 cm
The L-shape drawn on the board, with sides 5 cm, 2 cm, 3 cm, 4 cm, 2 cm and 2 cm going around. What is the distance all the way around? — 18 cm

Teacher Notes

This is the practice round — pupils take turns at the board, check each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

Each rectangle: ask which sides are hidden, and how the pupil found them, before they add.
Before the L-shape, sketch and label it on the board exactly like the L-shape in Watch and Notice: mark each of the six sides 5, 2, 3, 4, 2, 2 so pupils have a real figure to trace and count. Count every side aloud as a class first (there are six), then add them one at a time so none is missed or counted twice.

+15 XP

6 - What Did We Notice? ~3 mins

MATHS TALK

If you know the long side and the short side of a rectangle, why do you not need to measure the other two sides at all?

Teacher Notes

Listen for pupils naming the two pairs of equal sides as the reason — the top always matches the bottom, and one end always matches the other. Revoice a strong answer: so two numbers really tell you all four. Head off the idea that this works for any shape: it holds for rectangles because opposite sides match, but not for the L-shape, where you did have to check every side.

+10 XP

7 - What's Next ~2 mins

What we learned today

A rectangle has two pairs of equal sides, so once you know two sides you already know the other two.
To find the perimeter, fill in any missing sides first, then add every side once.
An L-shape has more than four sides, so you count and add each one carefully.

Coming up

Next time we look at two shapes that cover the same amount of space but have very different distances around, and we start to see that area and perimeter are not the same thing.

Teacher Notes

Keep this brief — a quick recap of the two-pairs-of-sides idea and a one-line look ahead to comparing area and perimeter.

+5 XP

Pupil practice

Module 5 · Area, Perimeter and Volume Measures

Lesson 54 · Perimeter of Rectangles and Simple Shapes

Download Activity Book page (PDF)

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This lesson is copyright of Coding Ireland 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~9 mins

Shape Measurer

3 - Try It Together ~10 mins

Find the missing sides, then the perimeter

4 - Draw the Rectangle in Your Copy ~3 mins

5 - Class Challenge ~8 mins

6 - What Did We Notice? ~3 mins

7 - What's Next ~2 mins

What we learned today

Coming up

Unlock the full learning experience

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