MathGrade 3ages 8–9
📏 cm and metre
"100 centimetres in a metre" sounds, to a child, like proof that a centimetre is the bigger unit — because 100 is a bigger number than 1 — and that backwards logic is the whole mistake.
Try this lesson freeNo card needed. The first lesson is free.
Download on the App StoreWhat your child learns
- One metre = 100 centimetres.
- A cm is smaller than a metre. You need a hundred of them.
- A pencil is measured in cm. A room in metres.
- A big number of small units is still not a big unit.
The mistake most kids make here
Thinking cm is bigger than a metre because 100 looks big.
Try one question
One metre = how many cm?
1 m
- 10
- 100
- 1000
Answer
100 — Yes. A hundred centimetres exactly fill a metre.
How Maya teaches it
Your child tells Maya what they did in class. She teaches this topic at her desk, asks questions with tappable answers, and when the answer is wrong she explains why that answer was tempting instead of saying “try again”. She remembers what did not land and comes back to it.
Your child has it when they can: How many cm in a metre?
How to help at home
This is really a lesson about units, not about length: a bigger count of small things is still not a big thing. Children who trust "bigger number equals bigger amount" without checking what's being counted fall into this every time.
Get a tape measure or ruler and measure something short in centimetres — a pencil, a hand — and something long in metres, like the hallway or the yard. Say the numbers out loud for each, then ask which unit you'd use to measure a whole room.
When your child says a centimetre must be bigger "because 100 is a big number", say: "look at how small one centimetre actually is on the ruler" and let them see the unit itself, not just the count of it.
A sign they've got it: they can say, before measuring, whether something should be measured in cm or metres — a pencil in cm, a room in metres — based on the size of the thing, not the size of the number.
Questions parents ask
- Why does my child think a centimetre is bigger than a metre?
- Because "100" looks like a big number, and children often haven't yet separated the size of a number from the size of the unit it's counting. Measuring real objects with a ruler and a tape measure side by side is what fixes it.
- When do children usually learn metric units like cm and metres?
- Most curricula introduce length units around age 8–9, alongside basic measuring with a ruler or tape measure.
- What household objects are good for practicing cm versus metres?
- A pencil, a hand, or a shoe work well for centimetres; a hallway, a door, or the garden work well for metres — pick one of each and measure both in the same sitting so the contrast is obvious.
- How does Maya teach length units?
- Maya asks which unit fits an object before asking for a measurement, so the child practices judging size first rather than only computing with numbers.
No card needed. The first lesson is free.
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