Primary 3 · Mathematics
P3 length, mass and volume
The units change, but the trap is one thing: converting by the wrong factor, or the wrong way. Most marks here are lost turning 3 km into 300 m instead of 3000 m. Here is the rule that fixes the direction and the factor at once.
- Level
- P3
- Usually taught
- Term 3
- Covers
- 1 concept
What changes at P3
In Primary 3, your child measures length, mass and volume, each in two units: kilometres and metres, kilograms and grams, litres and millilitres. The new skill is converting between them, and every pair works the same way.
The one thing to get right is the direction. To a smaller unit there are more of them, so the number goes up; to a bigger unit it goes down. Get both the direction and the factor right and every conversion follows the same rule.
The sentence that costs the mark
A road is 3 km long. How long is it in metres?
What the working usually does
× 1003 km=300 m
Uses 100, treating kilometres to metres like metres to centimetres. But 1 km is 1000 m, not 100, so this comes out ten times too small.
What the question asked
× 10003 km=3000 m
1 km is 1000 m, so 3 km is 3 lots of 1000. A smaller unit means more of them, so the number goes up.
300 is a tidy, believable number, which is exactly why it slips past. Two things fix it: the exact factor, 1 km is 1000 m, and a sense-check, a smaller unit always gives a bigger number. A 3 km road is a long way, so it has to be thousands of metres.
| Point | Value |
|---|---|
| 0 | 0 |
| 1000 m | 1000 |
| 2000 m | 2000 |
| 3000 m | 3000 |
The same question, worked properly
Which way are you converting?
km to m is a smaller unit, so more of themMetres are smaller than kilometres, so a length in metres is a bigger number than the same length in kilometres. The number must go up. That settles the direction before any arithmetic.
Multiply by 1000, not 100
3 × 1000, not 3 × 100This is the step that fails. The exact factor is 1 km = 1000 m. Using 100 gives 300, which looks fine but is ten times too small. Mixing it up with the metres-to-centimetres factor is where the mark is lost.
Check it makes sense
3 km = 3000 mA 3 km road is a long way, so thousands of metres is right. The same rule works for mass and volume: 3 kg = 3000 g, and 3 ℓ = 3000 ml.
How can you tell it has clicked?
Give a conversion each way: how many metres in 2 km, and how many kilometres in 5000 m. A child who makes the number bigger going to metres and smaller going to kilometres has the direction. One who divides both ways, or reaches for 100, has not.
The quickest tell is the sense-check. Before working, ask whether the answer should be bigger or smaller than what you started with. A child who can say which way it goes rarely gets the direction wrong.
One thing to try tonight
A 1-litre bottle holds a thousand millilitres, and a 1 km walk is a thousand metre-long strides. The small unit always piles up into a big number. Pour or pace it out once and the size of the gap between the two units stops being abstract.
When your child converts, ask one question first: "more of the small unit, or fewer?" Naming the direction out loud stops the divide-instead-of-multiply slip before the arithmetic even starts.
What your child will meet in this topic
- Length in km
Most Singapore schools introduce this in Term 3, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
These conversions run straight into P4 and P5, where lengths, masses and volumes sit inside longer word problems and must be in the same unit before you can add or compare them. A child fluent here does that step without thinking about it.
The "smaller unit, bigger number" idea is also the groundwork for decimals and place value: 3.2 km as 3200 m is the same move, one step on. Getting the direction solid now pays off across measurement for years.
More in Primary 3
Other topics in the same year, so you can see what sits either side of this one.
Struggling with the working rather than the topic itself? How to help your child with problem sums
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.