How to teach converting units of length
Grade 2 to Grade 6
Converting length means writing the same distance in a different unit, such as changing metres to centimetres. In the metric system the length units mm, cm, m and km scale by powers of ten, so every conversion is a multiply or divide by 10, 100 or 1000. Knowing the size of each unit and which way to convert is the whole skill.
How to teach it
- Anchor each unit to something real: a millimetre is about the thickness of a coin, a centimetre a fingernail's width, a metre a big stride, a kilometre a short walk.
- Learn the key relationships: 10 mm in a cm, 100 cm in a m, 1000 m in a km.
- Decide the direction: converting to a smaller unit multiplies (the number gets bigger), converting to a larger unit divides (the number gets smaller).
- Sense-check the size: 2 m written in centimetres should be a bigger number (200 cm), not a smaller one.
- Progress to comparing and ordering lengths given in mixed units by converting them to the same unit first.
Worked example
Convert 250 cm to metres: 100 cm = 1 m a larger unit, so divide 250 / 100 = 2.5 m
Common mistakes
- Multiplying when you should divide, so the answer is the wrong size.
- Using the wrong relationship (100 cm in a metre, not 10).
- Moving the decimal point the wrong number of places.
- Dropping the unit off the answer, so 250 becomes meaningless.
Frequently asked questions
What is converting units of length?
Converting length means writing the same distance in a different unit, such as changing metres to centimetres. In the metric system the units mm, cm, m and km scale by powers of ten, so every conversion is a multiply or divide by 10, 100 or 1000.
What age or grade is converting length taught?
Converting units of length is usually taught from Grade 2 to Grade 6. Students first anchor each unit to something real, learn the key relationships, then convert single units and progress to comparing and ordering lengths given in mixed units.
When do you multiply and when do you divide converting length?
Converting to a smaller unit multiplies, so the number gets bigger, and converting to a larger unit divides, so the number gets smaller. Changing 2 metres to centimetres multiplies by 100 to give 200, because centimetres are smaller. Sense-checking the size guards against getting this backwards.
What are the key length relationships?
The essential facts are 10 millimetres in a centimetre, 100 centimetres in a metre, and 1000 metres in a kilometre. These three relationships drive every length conversion, so it is worth learning them securely. Using the wrong one, such as 10 centimetres in a metre, causes errors.
Why does my child convert length the wrong way?
The usual error is multiplying when they should divide, or the reverse, so the answer is the wrong size, along with using the wrong relationship or moving the decimal point the wrong number of places. Checking that 2 metres in centimetres should be a bigger number catches most slips.
How do you help children picture each length unit?
Anchor each to something real: a millimetre is about the thickness of a coin, a centimetre a fingernail's width, a metre a big stride, and a kilometre a short walk. These references make the units concrete and support the sense-checking that catches conversion errors.
What comes after converting units of length?
Students move on to comparing and ordering lengths given in mixed units, by converting them to the same unit first, and to converting other measures like mass and capacity. The same multiply-or-divide-by-powers-of-ten idea carries across the whole metric system.
Practise with free worksheets
Printable worksheets with answer keys that are never wrong.