How to teach metric measurement and unit conversion

Grade 2 to Grade 5

Metric measurement uses units that scale by powers of ten, so converting between them means multiplying or dividing by 10, 100 or 1000. Length runs mm, cm, m, km; mass runs g, kg; capacity runs mL, L. Knowing the size of each unit and which way to multiply is the whole skill.

How to teach it

1. Fix the size of each unit with a real reference: a centimetre is about a fingernail's width, a metre is a big stride, a kilogram is a bag of sugar.
2. Learn the key relationships: 10 mm in a cm, 100 cm in a m, 1000 m in a km, 1000 g in a kg, 1000 mL in a L.
3. Decide the direction: going to a smaller unit multiplies (the number gets bigger), going to a larger unit divides (the number gets smaller).
4. Check the answer is sensible: 3 m as centimetres should be a bigger number (300 cm), not a smaller one.
5. Move to two-step problems and adding mixed units once single conversions are secure.

Worked example

Convert 3.5 m to centimetres:

   1 m = 100 cm
   a smaller unit, so multiply
   3.5 x 100 = 350 cm

Common mistakes

• Multiplying when you should divide, or the reverse, so the answer is the wrong size.
• Using the wrong relationship (there are 100 cm in a metre, not 10).
• Moving the decimal point the wrong number of places.
• Forgetting to write the new unit on the answer.