Metres, Centimetres, Kilograms: Where Unit Conversions Go Wrong

23 September 2026 · by Larry

He can multiply by 100 in his head. Ask him cold and he will tell you there are 100 centimetres in a metre. Then the paper comes back and 3.5 m has become 35 cm, and a question worth three marks has gone for a reason that has nothing to do with multiplication.

Conversions are one of the quietest sources of lost marks in primary maths, because they almost never look like the topic. They sit inside a question about something else — a length, a recipe, a parcel, a journey — and by the time the answer is wrong, the conversion is three steps back and invisible.

Why it slips

Three reasons, and none of them is not knowing the facts.

The numbers move in two directions and the child is only sure of one. Going from metres to centimetres, you multiply. Going back, you divide. Most children learn one direction properly — usually the one the first worksheet used — and then work out the other at speed, in their head, in the middle of a longer question.

The decimal point does the work now. By the time units appear with decimals, two skills are being tested at once: knowing the relationship, and moving a decimal point without losing track of it. A child shaky on decimals will look as though he does not know his conversions when the real gap is one place further back.

Nothing in the question warns him. A word problem gives one measurement in metres and another in centimetres and expects him to notice. There is no instruction saying "convert first". Noticing is the skill, and it is rarely the one being practised.

The four places marks actually go

1. Mixed units in the same question. "A ribbon 2 m long. He cuts off 45 cm." Everything about that sentence invites him to work with 2 and 45. The fix is mechanical: before any calculation, make him rewrite every measurement in the same unit, on its own line. Not in his head — on the paper.

2. Multiplying when he should divide. The check that works is not a rule, it is a question: am I going to end up with more of them or fewer? Centimetres are smaller than metres, so a length in centimetres is a bigger number. If his answer came out smaller, something has gone the wrong way. That single sentence catches most of it, and it keeps working when he meets grams and kilograms, millilitres and litres.

3. The answer is right but the unit is missing or wrong. This is the most annoying one to look at as a parent, because the maths was correct. Marks still go. Building the habit of writing the unit into every line of working — not just the final answer — is what stops it, and it is the same habit that saves marks in the other places children lose them for reasons that are not the maths.

4. He converted at the end instead of the start. Converting first, once, is one operation. Converting at the end means converting whatever came out of the sum, and if there are two or three measurements it means doing it more than once. More chances to slip, for nothing gained.

What helps at home

Not more conversion drills. If he can convert when asked directly but not inside a question, drills practise the part that already works.

What helps is making the relationship physical and fixing it to something he can see:

• A metre rule with centimetres on it. One hundred small marks in one long metre. He can count a few and believe the rest. Abstract until he has seen it once.
• The kitchen scale. Put a 500 g pack on it and ask what it would say in kilograms. Then ask him to find something that weighs about 2 kg, and to say what that is in grams before he checks.
• A measuring jug. One litre, marked in millilitres. Pour 250 ml four times.

Each of those takes two minutes and does something no worksheet does: it gives the numbers a size. A child who has held 500 g does not write 5 kg by accident.

The five-minute check tonight

No worksheet needed. Ask four things and watch how he answers, not just what he answers.

• "Is 250 cm longer or shorter than 3 m?" Listen for whether he converts before comparing, or guesses from the bigger-looking number.
• "Write 1.4 kg in grams." Then, immediately, "write 1400 g in kilograms." The second one is the direction most children are less sure of.
• "A bottle holds 2 l. She pours out 350 ml. How much is left?" Watch whether the first thing he writes is a conversion. If he starts subtracting straight away, that is the gap.
• "Will the number get bigger or smaller?" Ask this before he calculates anything. If he can answer it every time, he will catch his own mistakes for the rest of primary school.

If all four are comfortable, the topic is fine and the lost marks came from somewhere else — often the question itself rather than the arithmetic, which is the pattern in when the sums are fine but the story is not.

What does not help

Teaching it as "move the decimal point two places". It is quick, it works, and it is the first thing to fall apart under exam pressure, because there is nothing underneath it to check against. A child who has only the trick has no way of knowing whether he moved it the right way.

And telling him to be more careful. He was careful. He carefully converted in the wrong direction, which is a different problem with a different fix — the same distinction as in what careless mistakes usually are.

The short version

• Conversions rarely fail because the facts are unknown. They fail inside longer questions, where nothing says "convert first".
• Most children are sure of one direction and work the other out at speed. Practise the unsure direction.
• Rewrite every measurement in the same unit, on paper, before calculating anything.
• The check that always works: will the number get bigger or smaller?
• Write the unit on every line of working, not just the answer.
• Convert at the start, once — not at the end, several times.
• Two minutes with a ruler, a kitchen scale and a measuring jug beats twenty more conversion sums.

If the underlying multiplying and dividing by 10, 100 and 1000 is the slow part, fix that first — otherwise every conversion question is two problems at once.