The idea, step by step
Place the protractor
Put the centre mark exactly on the vertex. Line up its zero line with one ray: this is your baseline. Check that the ray runs through the centre line.
If a ray is short, extend it with a ruler. That changes its length, not the angle.
Start at the correct zero
Many protractors have two scales. Start at zero on your baseline and count towards the other ray. Do not simply choose the easiest number to see.
Estimate first. An acute opening cannot correctly measure 130 degrees. Our single-scale diagrams start from the right: for a left baseline, subtract the displayed reading from 180.
A common trap
Either scale gives an acceptable answer.
Instead: Use the scale beginning at zero on your chosen baseline.
See the thinking
Right baseline
Read the angle shown.
- The baseline points right, where the displayed scale starts at zero.
- The other ray crosses 40. Its acute appearance agrees.
Left baseline
Read the angle from the left baseline.
- The second ray reads 130 from the right.
- Count from the left: 180 − 130 = 50.
Check your understanding
A right baseline and ray at 60 on the right-zero scale make what angle?
Show answer
60 degrees
Start at the baseline zero and read towards the other ray.
Your turn
6 questions · 9 marks · Work at your own pace.
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; 180 − 140 = 40.
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 ↗