Perimeter: Adding Sides up

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Sketch and Sum in Your Copy
5 Class Challenge
Pupil practice

1 - Getting Started ~4 mins

Imagine a garden shaped like a pentagon, with five straight sides: 3 m, 5 m, 4 m, 6 m and 7 m. To build a fence all the way around it, how much fencing would you need?

Key point

That distance all the way round is what we call the perimeter. Today we'll figure out exactly how to find it for any shape, straight-edged or wonky, simple or stretched out.

Teacher Notes

Sketch the irregular pentagon on the board with sides labelled 3, 5, 4, 6, 7 in metres as the class settles. Take three hands-up answers to the fencing question, not open call-outs; accept any reasonable response (most will sum to 25) and acknowledge the strategy without grading. The 'add them up' move is exactly what the lesson formalises in the next step.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~7 mins

Illustration for Watch and Notice Watch as we walk the boundary of three rectangles on the board. Each one shows the perimeter readout at the bottom: the running total of all four sides.

6 m × 4 m rectangle

A long-side, short-side, long-side, short-side trip round the edge: 6 + 4 + 6 + 4 = 20 m.

5 m × 5 m square

All four sides are the same: 5 + 5 + 5 + 5 = 20 m. Same perimeter as the first one, but a very different shape.

8 m × 3 m long rectangle

Stretched out: 8 + 3 + 8 + 3 = 22 m. Long and thin uses up more fencing than tall-and-square does for the same floor space.

A five-sided shape

Now a shape that isn't a rectangle. We do exactly the same thing — add up every side: 6 + 4 + 5 + 5 + 4 = 24 m.

An L-shape

This one has six sides. Walk all the way round and add every one: 3 + 3 + 4 + 2 + 7 + 5 = 24 m. The same trick works no matter how many sides a shape has.

Teacher Notes

Point at each rectangle in turn and say each addition aloud: 'six plus four is ten, plus six is sixteen, plus four is twenty; perimeter is twenty metres.'

Lock the 'same perimeter, different shape' moment hard. The 6×4 and the 5×5 both come out at 20 m. Do not move on until the class can tell you that walking the edge once and adding every side gives the perimeter, no matter the shape; ask the question and take a couple of answers.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~12 mins

We're going to measure three small objects in the room, all in centimetres: the Maths book, an A4 page from your folder, and a copybook. One volunteer at a time comes up to measure with the tape; another pupil enters the dimensions on the board. The class predicts the perimeter before the readout shows it.

Classroom objects

Teacher Notes

This round is for talking it through together; pupils take turns at the board and the class agrees or corrects out loud.

Use a single tape measure (or each pupil's own ruler) and pass it to the volunteer for each object. Send three pupils in turn: one measures the Maths book, one measures an A4 page, one measures a copybook. The rest of the class watches the IWB as the shape-measurer rectangle is sized to match each set of dimensions.

Push for measurement to the nearest centimetre. After each measurement, say the dimensions aloud and run the prediction beat before the readout: 'the book is 26 cm by 19 cm; what's the perimeter?' Take a couple of answers before tapping the shape-measurer readout to confirm. Repeat for the A4 page and the copybook.

Watch for the common slip: pupils calling the longer side once and the shorter side once instead of twice each. Every side has a partner side on a rectangle; count both.

+15 XP

4 - Sketch and Sum in Your Copy ~4 mins

COPYBOOK MOMENT

In your maths copy, sketch each of the three objects we just measured: the Maths book, the A4 page, and the copybook. Label every side with the measurement we agreed on. Underneath each sketch, write the sum of all four sides and circle the perimeter total.

Teacher Notes

Walk the row glancing for two things: every side labelled (no missing sides), and the sum showing every side once. This is whole-class copybook practice, not marking; if a sketch shows only three sides labelled, point at the missing side and move on. If a pupil is still on the first sketch when time is up, let them finish that one and skip the rest rather than rushing the addition.

+10 XP

5 - Class Challenge ~12 mins

Now it's your turn, in pairs. Measure all the way round some everyday objects and the shapes on your sheet, and work out the perimeter of each — the distance all the way round.

Measure each side in centimetres — the metre stick or tape for big things, your ruler for the sheet shapes.
Sketch it in your maths copy and label every side with its length.
Add all the sides together and write the sum underneath.
Circle the perimeter and write cm beside it.
Swap with your partner and check each other's totals.

Measure your desk top and the cover of your Maths book, then the four shapes on the Perimeter shapes sheet — a triangle, a five-sided shape, an L-shape and a hexagon. The shapes with more than four sides are where adding every side really matters.

Stretch

Measure all four shapes on the sheet, then find a shape in the room with even more sides and add up all of its sides too.

Hands-on Task

Teacher Notes

Before the lesson: print the Perimeter shapes sheet, one per pair — print at 100% (actual size), not 'fit to page', so the sides measure true. Have metre sticks or tapes and a few 30 cm rulers ready to share.

This is the practice round: pairs measure and add at their seats while you circulate. Keep it brisk. Watch for two things — measuring from the very end of the object rather than the 0 mark, and dropping a side on the shapes with more than four sides (Shapes 3 and 4 are the ones that make 'add all the sides' matter). For a quick check, the four sheet shapes come to 24, 24, 28 and 28 cm.

Pair a less secure pupil with a more confident partner; the partner-check at the end is the quick self-mark.

+15 XP

Pupil practice

Module 4 · Measures: Length, Mass, Capacity, Area, Volume Measures

Lesson 49 · Perimeter: Adding Sides up

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Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~7 mins

6 m × 4 m rectangle

5 m × 5 m square

8 m × 3 m long rectangle

A five-sided shape

An L-shape

3 - Try It Together ~12 mins

Classroom objects

4 - Sketch and Sum in Your Copy ~4 mins

5 - Class Challenge ~12 mins

Hands-on Task

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