First decide what is being measured. Length (how long) uses millimetres, centimetres, metres or kilometres. Mass (how heavy) uses grams or kilograms. Capacity (how much something holds) uses millilitres or litres. Then use a benchmark to picture the size of one unit: a paperclip is about 1 gram, a bag of sugar is 1 kilogram, a milk carton holds 1 litre, a teaspoon holds about 5 millilitres, a big step is about 1 metre. Ask whether the object is about as big as a few of the benchmark, or hundreds of them.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- mass
- how heavy something is
- capacity
- how much something can hold
- benchmark
- a familiar object used to picture the size of a unit
Worked examples
Which unit would you use to measure the mass of an apple: grams, kilograms, millilitres or centimetres?
- 1.Mass uses grams or kilograms.
- 2.An apple is much lighter than a bag of sugar, so use grams.
= grams
Which is the most sensible capacity for a bucket: 10 mL, 10 L, 10 kg or 10 m?
- 1.Capacity uses mL or L.
- 2.A bucket holds about 10 milk cartons.
= 10 L
A bag of sugar has a mass of about 1 kg. Which is the best estimate for the mass of a cat: 4 kg, 40 kg, 400 kg or 4000 kg?
- 1.A cat is about as heavy as a few bags of sugar.
- 2.40 kg is as heavy as a big child.
= 4 kg
For parents
Method: Match the attribute, then picture a benchmark
What to say: Are we measuring how long, how heavy or how much it holds? How many milk cartons (or sugar bags) would it be?
Common mistakes:
- Knows which kind of unit to use but picks one that is far too big or too small (kilograms for an apple).
- Uses a unit for a different attribute, such as kilograms for how much a container holds.
- Thinks a large object must be heavy, so picks kilograms for something big but light (a pillow, a balloon).