Understand the idea.
Choose a useful unit
First decide which attribute is being measured. Length uses units such as millimetres and metres; mass uses grams or kilograms; capacity uses millilitres or litres. The unit must match the attribute.
Choose a unit suited to the size: centimetres suit a desk, while kilometres suit a road journey. Converting the unit changes the number used to describe the measurement, not the actual size.
Count intervals and match the scale
To read a scale, subtract neighbouring numbered labels and divide by the number of equal gaps between them. Count the intervals, not the tick marks. Each interval represents the same amount.
For length, align the object with zero or subtract its starting reading from its ending reading. Report only the detail the scale supports; an endpoint between marks may need an estimate rather than extra decimal places.
See the method in action.
Read the half-centimetre ruler
Find the object length in the diagram.
Worked solution
- 1
Find the size of one small interval.
Each labelled centimetre is divided into two equal gaps.
- 2
Read the right endpoint.
The object ends one half-centimetre beyond the 3 cm mark.
- 3
Subtract the starting reading.
The object begins at zero, so the endpoint reading is also its length.
3.5 cm
Find the gap size
A jug is labelled 200 mL and 400 mL, with 4 equal gaps between the labels. What does each gap represent?
Worked solution
- 1
Find the amount between the labelled marks.
The four gaps together cover the difference between the labels.
- 2
Share this amount among four equal gaps.
Equal spaces on this scale represent equal capacities.
- 3
Check by rebuilding the upper label.
Four 50 mL increases take the scale from 200 mL to 400 mL.
50 mL
Try it yourself.
Check your understanding
A scale goes from 10 kg to 20 kg in 5 equal gaps. What is one gap worth?
Show answer & reasoning
2 kg
The five gaps cover . Each gap therefore represents .
Keep practising
6 worksheet questions · 11 marks
Read the questions online
- Choose a useful metric unit for the distance between towns.1 mark
- Choose a useful metric unit for the mass of an apple.1 mark
- Convert 3 m to centimetres.2 marks
- A jug has 5 equal gaps from 0 mL to 500 mL. Find the size of one gap.2 marks
- A pencil begins at 2 cm and ends at 14 cm on a ruler. Find its length.2 marks
- A ruler has marks every 1 cm. Lee reports an exact length of 7.438 cm after only reading that ruler. Explain the problem.3 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Choose a useful metric unit for the distance between towns.1 mark
A distance between towns is large enough that kilometres give a useful number.
Kilometres (km) - Choose a useful metric unit for the mass of an apple.1 mark
Grams give a useful number for an object much lighter than a kilogram.
Grams (g) - Convert 3 m to centimetres.2 marks
Each metre is 100 cm.
.
300 cm - A jug has 5 equal gaps from 0 mL to 500 mL. Find the size of one gap.2 marks
.
100 mL - A pencil begins at 2 cm and ends at 14 cm on a ruler. Find its length.2 marks
Subtract the starting reading: cm.
12 cm - A ruler has marks every 1 cm. Lee reports an exact length of 7.438 cm after only reading that ruler. Explain the problem.3 marks
Its marks show whole centimetres.
Extra decimal digits do not make the measurement more accurate.
The ruler does not show thousandths of a centimetre, so it cannot support that exact reading.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics 2026
Units/precision; regular/irregular regions; boundary and area problems.
Official curriculum source ↗