The idea, step by step
Choose a scale that suits the task
A ruler marked in millimetres is better for a small craft piece than a metre stick marked only in centimetres. A measuring jug with 50 mL gaps cannot show a 1 mL reading exactly. Smaller marked gaps usually allow a finer reading.
The instrument must also cover the amount needed. A 100 mL cup cannot measure 2 L in one fill. Decide whether repeated fills are practical or whether a larger suitable jug would reduce counting mistakes.
Measure carefully and check
Line an object's starting edge up with zero, not with the end of a ruler if they differ. If the object begins at another mark, subtract the starting reading from the finishing reading. Keep the ruler straight and look directly at the scale.
Repeat a measurement if it seems surprising. Readings of 28, 28 and 29 cm are close; 82 cm is probably a mistake for the same object. Report what the instrument supports, and avoid claiming an exact fine measurement from a coarse scale.
A common trap
A strip finishes at 9 cm, so it is 9 cm long even though it starts at 2.
Instead: Length is the difference between start and finish: 9 − 2 = 7 cm.
See the thinking
Measure from an offset
A straight strip begins at the 2 cm mark and finishes at the 9 cm mark. How long is it?
- The end reading is not the length because the start is not zero.
- Subtract the start from the finish: 9 − 2 = 7.
- Check that seven 1 cm gaps lie between 2 and 9.
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?
- Both jugs have enough capacity.
- A has smaller marked gaps: 10 mL rather than 100 mL.
- 150 mL is marked by A's 10 mL divisions, while B would require estimating between 100 and 200.
Check your understanding
A strip runs from 3 cm to 11 cm. What is its length?
Show answer
8 cm
11 − 3 = 8 cm; count the gaps between the marks.
Your turn
6 questions · 10 marks · Work at your own pace.
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 1 L = 1000 mL. 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
16 − 1 = 15 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 1 L = 1000 mL. How many full 250 mL cups are needed, and what must you count carefully?2 marks
1000 ÷ 250 = 4.
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 ↗