Understand the idea.
Choose a scale that suits the task
Choose an instrument with enough capacity or range for the task and marks fine enough for the reading you need. A jug with 50 mL gaps gives less detail than one with 10 mL gaps; it cannot show a 1 mL change directly.
Check both features. A small cup may have fine marks but require many refills to measure a large amount. A larger suitable jug may make the job easier and reduce mistakes when counting fills.
Measure carefully and check
Line the start of an object up with the ruler’s zero mark, which may differ from its physical end. If it begins at another mark, its length is the finishing reading minus the starting reading. Keep the ruler straight.
Look directly at the scale and repeat a surprising reading. Measurements of 28, 28 and 29 cm are close; 82 cm would need checking. Fine marks help, but careful positioning and reading are still necessary.
See the method in action.
Measure from an offset
A straight strip begins at the 2 cm mark and finishes at the 9 cm mark. How long is it?
Worked solution
- 1
Identify the two ruler readings.
The strip does not start at zero, so the finishing label alone is not its length.
- 2
Subtract the starting reading.
Length measures the distance between the two ends of the strip.
- 3
Check by counting on.
Seven 1-centimetre gaps take us from the starting mark to the finishing mark.
7 cm.
Explain an instrument choice
You need about 150 mL of water. Jug A has 10 mL gaps; jug B has 100 mL gaps. Both hold at least 500 mL. Which gives the finer reading?
Worked solution
- 1
Check that both jugs hold enough.
Both stated capacities cover the amount needed, so capacity does not rule either jug out.
- 2
Compare the scale divisions.
Jug A divides the liquid amount into smaller marked steps.
- 3
Locate the target reading.
On A, 150 mL is a 10-millilitre division; on B it lies between 100 and 200 mL and needs estimating.
- 4
Choose the finer scale.
Jug A gives the finer reading because its smaller gaps show the required amount more clearly.
Jug A gives the finer reading.
Try it yourself.
Check your understanding
A strip runs from 3 cm to 11 cm. What is its length?
Show answer & reasoning
8 cm
The strip spans the gap from 3 to 11, so subtract: . Counting eight 1-centimetre gaps gives the same length.
Keep practising
6 worksheet questions · 10 marks
Read the questions online
- Choose a suitable instrument for the mass of a small packet: a ruler or kitchen scales.1 mark
- A pencil runs from 1 cm to 16 cm on a ruler. Find its length.1 mark
- For measuring 125 mL, which jug has finer markings: 5 mL gaps or 50 mL gaps?2 marks
- One book is measured four times: 24, 24, 25 and 42 cm. Which reading should be checked first? Explain.2 marks
- A ruler shows only whole centimetres. A learner reports a length of exactly 8.367 cm. Explain the problem.2 marks
- A container needs 1 L of water. Use . How many full 250 mL cups are needed, and what must you count carefully?2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Choose a suitable instrument for the mass of a small packet: a ruler or kitchen scales.1 mark
The quantity is mass, not length.
Kitchen scales. - A pencil runs from 1 cm to 16 cm on a ruler. Find its length.1 mark
cm.
15 cm - For measuring 125 mL, which jug has finer markings: 5 mL gaps or 50 mL gaps?2 marks
Smaller gaps allow a more detailed reading.
125 is a multiple of 5 but lies between the 50 mL marks.
The jug with 5 mL gaps. - One book is measured four times: 24, 24, 25 and 42 cm. Which reading should be checked first? Explain.2 marks
It is far from the three readings near 24–25 cm.
Check the ruler's position and repeat; do not simply assume the true length.
42 cm. - A ruler shows only whole centimetres. A learner reports a length of exactly 8.367 cm. Explain the problem.2 marks
A position between 8 and 9 cm can only be estimated more roughly from this scale.
A suitably finer instrument would be needed.
The scale does not support that many decimal places. - A container needs 1 L of water. Use . How many full 250 mL cups are needed, and what must you count carefully?2 marks
.
Keep a record so no fill is missed or counted twice.
4 cups; count all four complete fills.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics 2026
Length/mass/capacity/temperature; scaled instruments; partial/unmarked intervals; suitable units and accuracy.
Official curriculum source ↗