Understand the idea.
Place the protractor
Place the protractor centre mark on the vertex, where the two rays meet. Align its zero line with one ray. This ray is the baseline from which the angle will be counted.
If a ray is too short to reach the scale, extend it straight with a ruler. Keep the protractor centred and aligned: moving its centre away from the vertex changes the reading.
Start at the correct zero
On a double-scale protractor, choose the scale whose zero lies on your baseline. Follow that scale towards the other ray. The two numbers at a mark measure from different starting sides.
Estimate the opening before reading: an acute angle must be below . Our diagrams show a right-zero scale. With a left baseline, subtract its displayed reading from to measure from the other end.
See the method in action.
Right baseline
Read the angle shown.
Worked solution
- 1
Find the zero on the baseline.
The baseline points right, matching the zero of the displayed scale.
- 2
Read where the second ray crosses.
Counting from the right-hand zero reaches the mark labelled 40.
- 3
Compare with the visible opening.
The measured angle is acute, agreeing with the diagram.
40 degrees
Left baseline
Read the angle from the left baseline.
Worked solution
- 1
Identify which side starts the turn.
The left baseline means the angle must be measured from the left end of the semicircle.
- 2
Read the other ray on the displayed scale.
The diagram labels this position by counting from the right, not from the left baseline.
- 3
Subtract from the straight-angle total.
The left and right readings together cover the semicircle.
- 4
Check the angle class.
The left-hand opening is acute, so the 50-degree answer fits.
50 degrees
Try it yourself.
Check your understanding
A right baseline and ray at 60 on the right-zero scale make what angle?
Show answer & reasoning
60 degrees
The baseline starts at the right-hand zero. The ray crosses 60 on that same scale, so the angle is .
Keep practising
6 worksheet questions · 9 marks
Read the questions online
- Read this angle.1 mark
- Read this angle.1 mark
- Read from the left baseline.2 marks
- A ray is halfway between 70 and 80 degrees on the correct scale. Find the angle.1 mark
- A learner puts the protractor edge at the vertex but not its centre. Explain the problem.2 marks
- An acute angle gives readings 50 and 130 on the two scales. Which is reasonable? Explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Read this angle.1 mark
Use the right baseline zero and read 30.
30 degrees - Read this angle.1 mark
Use the right baseline zero and read 110.
110 degrees - Read from the left baseline.2 marks
The ray reads 140 from the right; .
40 degrees - A ray is halfway between 70 and 80 degrees on the correct scale. Find the angle.1 mark
Halfway between 70 and 80 is 75.
75 degrees - A learner puts the protractor edge at the vertex but not its centre. Explain the problem.2 marks
The centre mark must match the vertex, not an arbitrary edge.
The reading may be wrong. - An acute angle gives readings 50 and 130 on the two scales. Which is reasonable? Explain.2 marks
Acute means less than 90, whereas 130 is obtuse.
50 degrees
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Year 5 angle estimation and measurement using a protractor.
Official curriculum source ↗