Length: Centimetres and Millimetres

Warning: Your screen size is quite small for viewing this. Rotate your device into landscape mode if you can.

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

1 - Getting Started ~4 mins

Illustration for Getting Started Look very closely at your own ruler, right between the big number 3 and the big number 4. Can you see the tiny little marks crowded in between them? How many little gaps do you think you could count from one big number to the next?

Teacher Notes

Hold up a real ruler and zoom in on the IWB photo onto the marks between 3 cm and 4 cm. Take three hands-up guesses before anyone counts — the surprise is that there are exactly ten little gaps every single time.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~8 mins

Illustration for Watch and Notice Watch as we read one length on the ruler together, very carefully. First we find the last whole centimetre the object reaches, then we count on in those little millimetre marks to the very end.

Worked example

The pencil on the board passes the big number 4 and goes five little marks further. So it reaches 4 cm and 5 mm.

Key point

Now let us write that same length a second way, in millimetres only. Each centimetre is 10 mm, so 4 cm is 40 mm. Then we have 5 more little marks, so 40 + 5 makes 45 mm altogether. We always do the same thing: multiply the centimetres by 10, then add on the extra millimetres.

Teacher Notes

Use the measurement-rulers display showing the pencil at 4 cm 5 mm. Point first to the last whole centimetre (the 4), then count the little marks aloud one at a time up to 5.

Emphasise that ten little marks fill one whole centimetre, so five little marks is half a centimetre.
Say the length aloud as four centimetres and five millimetres.
Then build the conversion slowly: 4 centimetres is 4 lots of 10, which is 40 millimetres; add the 5 extra and we get 45 millimetres. Write 4 × 10 + 5 = 45 on the board so pupils see where 45 comes from, not just the answer.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~9 mins

Now we read a length on the board together. We look at the object on the ruler and read it first to the nearest whole centimetre, and then read it more carefully to the nearest millimetre.

Key point

Each time, we ask ourselves two questions: how many whole centimetres does it reach, and how many extra little marks past that?

Read to the nearest millimetre

Teacher Notes

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

Drive the measurement-rulers interactive in explore mode. Send a pupil up to drag the cursor to the end of the object on screen, and ask the class first how many whole centimetres? then how many extra little marks past that? Move the cursor to a new spot for each pupil so the class reads several different lengths.

Watch for pupils reading from the wrong end or starting the millimetre count from zero rather than from the last whole centimetre. Rotate a few pupils through.

+15 XP

4 - Write the Lengths in Your Copy ~3 mins

COPYBOOK MOMENT

In your maths copy, write 1 cm = 10 mm at the top of a fresh line. Then record three small objects, each with its length written two ways: once in centimetres-and-millimetres, and once in millimetres altogether. Remember the rule: multiply the centimetres by 10, then add on the extra millimetres.

Set out each object on its own line, like this:

rubber: 4 cm 5 mm = (4 × 10) + 5 = 45 mm
key: ___ cm ___ mm = ___ mm
crayon: ___ cm ___ mm = ___ mm

Teacher Notes

Walk the room glancing at whether pupils have converted correctly (4 cm 5 mm = 45 mm, not 405 mm). If anyone writes 405, point them back to the rule on the board: 4 × 10 + 5. This is whole-class copybook practice, not marking.

+10 XP

5 - Class Challenge ~9 mins

Now measure four small classroom items with your own ruler, reading each one to the nearest millimetre: a crayon, a rubber, a key and a lolly stick. Write each one as "___ cm ___ mm".

Stretch

For a stretch, also try a coin and a staple. These two are the trickiest to line up, so it is fine to read them to the nearest millimetre as best you can. Then find the difference in millimetres between your longest and your shortest item.

Hands-on Task

Teacher Notes

This round is the practice bank — real objects circulate around the room (or sit at stations) and pupils measure each one with their own rulers. Keep the pass-on rhythm brisk; the class confirms each reading aloud at the end.

Circulate and catch alignment slips on the spot — the most common is lining the object up against the ruler's edge instead of the zero mark, or counting millimetres from zero rather than from the last whole centimetre.

The crayon, rubber, key and lolly stick all land cleanly on the ruler, so they are the four core items everyone measures. Keep the coin and staple for the stretch only — they are short and fiddly to align.

Differentiation: pair a pupil who is less secure with a peer who can model lining up the zero carefully; stretch confident pupils with the coin, the staple and the longest-minus-shortest difference question.

+15 XP

6 - What Did We Notice? ~3 mins

MATHS TALK

When would millimetres tell us something that whole centimetres would miss? Think of a time it really matters to be that little bit more exact.

Teacher Notes

Listen for pupils naming real situations where small differences matter — fitting a battery, a key, a screw, a piece that must slot in exactly. Revoice: so two things can look the same number of centimetres long but be a few millimetres different, and sometimes those millimetres are the whole story.

+10 XP

7 - What's Next ~2 mins

What we learned

There are ten millimetres in every centimetre, shown by the little marks between the big numbers.
To read a length, find the last whole centimetre, then count on in millimetres to the end.
A length like 4 cm 5 mm can also be written as 45 mm: multiply the centimetres by 10, then add the extra millimetres (4 × 10 + 5 = 45).

Coming up

Next we look at measuring longer things, choosing between centimetres and metres so we pick the unit that fits.

Teacher Notes

Keep this brisk. Recap the 1 cm = 10 mm fact one more time before the class moves to the activity-book page.

+5 XP

Pupil practice

Module 4 · Measures: Length, Weight and Capacity Measures

Lesson 44 · Length: Centimetres and Millimetres

Download Activity Book page (PDF)

End of lesson

123learn · Online learning platform

Unlock the full learning experience

You're previewing this lesson. Get full access to this lesson and hundreds more — each one ready to teach, with interactive activities, printable resources and pupil progress tracking built in.

Hundreds of curriculum-aligned lessons

Interactive activities in every lesson

Printable resources & progress tracking

Get started

Copyright Notice
This lesson is copyright of 123Learn.ie 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

2 - Watch and Notice ~8 mins

3 - Try It Together ~9 mins

Read to the nearest millimetre

4 - Write the Lengths in Your Copy ~3 mins

5 - Class Challenge ~9 mins

Hands-on Task

6 - What Did We Notice? ~3 mins

7 - What's Next ~2 mins

What we learned

Coming up

Unlock the full learning experience

XP