Unit conversion: the 100 or 1000 mistake
Your child converts cm³ to litres by dividing by 100, or kg to g by multiplying by 100, and the answer is ten times off. Volume questions are where it costs the most, because the conversion usually comes in the middle of a 3 or 4 mark question and every step after it is wrong too.
What it looks like
A rectangular container measures 20 cm by 15 cm by 12 cm. It is completely filled with water. Ravi pours all the water into bottles that hold 1.5 ℓ each. How many bottles can he fill completely?
- Volume of water: 20 × 15 × 12 = 3600 cm³
- 3600 cm³ = 3600 ÷ 1000 = 3.6 ℓ
- 3.6 ÷ 1.5 = 2.4
- He can fill 2 bottles completely.
Why 24 bottles is wrong: The volume was divided by 100 instead of 1000, giving 36 ℓ, and 36 ÷ 1.5 = 24.
Why children do it
- Centimetres and metres use 100, so 100 becomes the default number for any conversion.
- Children learn the facts (1 ℓ = 1000 cm³, 1 kg = 1000 g) separately and mix them up under pressure.
- A wrong conversion still gives a reasonable-looking number, so nothing warns your child.
How to fix it at home
- Keep a small card with the five facts: 1 m = 100 cm, 1 km = 1000 m, 1 kg = 1000 g, 1 ℓ = 1000 ml, 1 ℓ = 1000 cm³.
- Ask your child: "is the new unit bigger or smaller?" Bigger unit, smaller number; smaller unit, bigger number.
- Use real objects: a 1.5 ℓ bottle at home holds 1500 ml, not 150.
- Have your child write the conversion on its own line so the marker, and your child, can see it.
Questions parents ask
Which conversions come up most in the PSLE?
Litres, millilitres and cm³ in volume questions, and kg and g in mass questions. Metres and centimetres appear too, but children get those wrong less often.
My child knows 1 ℓ = 1000 ml. Why the mistake?
Knowing a fact and choosing it in the middle of a long question are different. Mixed practice where the conversion is not announced builds the second skill.
Where it shows up
Other careless mistakes
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