Choosing and Converting Metric Units of Length

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

1 - Getting Started ~4 mins

Imagine I told you the corridor outside our classroom is 30,000 millimetres long, and your pencil is one fifty-thousandth of a kilometre long. Both of those are true! But would you ever actually say them that way?

Which unit would you really choose for the corridor? Which for your pencil? And why does picking the wrong one make a number look silly?

Teacher Notes

Take three hands-up answers, not open call-outs. Give five seconds of quiet think-time before any hands go up.

The tiny kilometre figure for the pencil is meant to sound absurd — pupils don't need to work it out, they just need to hear how clumsy it is. Listen for pupils naming metres for the corridor and centimetres for the pencil. Revoice: so we pick the unit that gives a number that is easy to say and picture.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

Illustration for Watch and Notice We line up the units as a ladder — km, m, cm, mm — and each rung is ten times the one below it. As we convert, the decimal point stays where it is; it is the digits that step into new columns. The point only appears to move.

2.5 m to 250 cm

Watch as the converter turns 2.5 metres into centimetres. There are 100 cm in every metre, so the digits step two places and we land on 250 cm.

1750 mm to 1.75 m

Now watch 1750 millimetres become metres. There are 1000 mm in a metre, so the digits step three places the other way, giving 1.75 m.

3.4 km to 3400 m

Watch 3.4 kilometres become metres. Each kilometre holds 1000 m, so 3.4 km is 3400 m.

1200 cm to 0.012 km in two jumps

This last one takes two jumps down the ladder. First 1200 cm becomes 12 m (100 cm in a metre, so divide by 100). Then 12 m becomes 0.012 km (1000 m in a kilometre, so divide by 1000). Watch each jump in turn.

Teacher Notes

Open by tracing the ladder on the board: km, m, cm, mm. Stress that the decimal point is fixed and the digits do the moving — this is the idea the whole lesson rests on. Walk each example aloud, one at a time.

2.5 m to cm — say each metre is 100 cm, so we multiply by 100. Point to the digits stepping two places.
1750 mm to m — there are 1000 mm in a metre, so we divide by 1000; the point appears to move three places.
3.4 km to m — a clean ×1000; this one usually feels easy.
1200 cm to km — the two-jump example. Do it as two separate moves: cm to m first, then m to km. This is the route pupils will need for the stretch in the Class Challenge, so name each jump clearly.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~7 mins

We work through three conversions together on the converter. For each one, the whole class decides first: are we going up the ladder or down, and do we multiply or divide? Say your answer aloud so we can agree or correct it. One pupil comes up to drive the converter while everyone else watches the digits and checks the result.

Watch out

Each conversion uses different units, so the pupil at the board changes both the start unit and the end unit before converting: 4.2 m to centimetres, then 6500 mm to metres, then 0.75 km to metres.

Convert these lengths together

Teacher Notes

This round is for talking it through together — a different pupil comes up for each conversion and the class agrees or corrects out loud.

Before each conversion, ask the class: are we going up the ladder or down — do we multiply or divide, and by how much? Remind the pupil at the board to set both units for each new prompt (the converter starts with both set the same, so nothing is pre-converted). Watch for the gut-instinct slip of moving the point only one or two places between mm and m.

+15 XP

4 - Write the Lengths Two Ways in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, write each of these lengths in two different units, side by side. Underline the unit you would actually choose to measure that real object.

your pencil
the classroom door height
the school yard length
the road to the nearest shop

For example: pencil — 18 cm or 180 mm (underline cm). If your teacher has handed out the printable conversion-ladder worksheet, use it to keep your two units lined up in their columns.

Teacher Notes

Walk the room glancing at which unit pupils underline and whether their two-unit conversion lines up. No marking — this is whole-class copybook practice, not assessment. The printable conversion-ladder worksheet (km – m – cm – mm columns with ÷10 / ÷100 / ÷1000 labels) gives weaker readers a scaffold; pupils without it simply rule the same columns in their copy.

+10 XP

5 - Class Challenge ~9 mins

Now we tackle fresh conversions, getting trickier as we go: 7.3 m to centimetres, 4200 mm to metres, 0.62 km to metres, and then the stretch — express 1250 cm in kilometres using the two-jump route we saw earlier.

Convert the length

Teacher Notes

This round is the practice bank — a pupil comes up for each conversion, checks each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

The final challenge (1250 cm to km) needs the two-jump route modelled in Watch and Notice — cm to m, then m to km. Let a confident pupil unpack it, then revoice the route. As each pupil works at the board, the watching class follows the digits and confirms the next jump aloud before it is solved.

+15 XP

6 - What Did We Notice? ~3 mins

MATHS TALK

Why does the decimal point seem to move three places between millimetres and metres, but only two places between centimetres and metres? What is happening to the digits each time?

Teacher Notes

Listen for pupils linking the number of places to the size of the step — three places for mm to m because there are 1000 mm in a metre, two places for cm to m because there are 100 cm in a metre. Revoice the strongest answer: the digits step one column for every ten, so a thousand means three columns. Head off the idea that the point itself moves — it is the digits shifting.

+10 XP

7 - What's Next ~3 mins

Key Takeaways

We choose the metric unit that gives a sensible-sized number for the real object.
Converting between mm, cm, m and km means multiplying or dividing by powers of ten.
There are 1000 mm in a metre, so the digits step three places between mm and m.

Coming Up

Coming up

Next we move from measuring along a line to measuring all the way around a shape, when we work out the perimeter of regular and irregular shapes.

Teacher Notes

Recap the ladder briefly: km, m, cm, mm, each step a power of ten. Confirm pupils can name a sensible unit for three different objects before moving on.

+5 XP

Pupil practice

Module 5 · Measures: Length, Area, Volume, Mass and Capacity Measures

Lesson 47 · Choosing and Converting Metric Units of Length

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Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~9 mins

2.5 m to 250 cm

1750 mm to 1.75 m

3.4 km to 3400 m

1200 cm to 0.012 km in two jumps

3 - Try It Together ~7 mins

Convert these lengths together

4 - Write the Lengths Two Ways in Your Copy ~3 mins

5 - Class Challenge ~9 mins

Convert the length

6 - What Did We Notice? ~3 mins

7 - What's Next ~3 mins

Key Takeaways

Coming Up

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