Primary 4 maths · Geometry · About 10 minutes
Measure angles with a protractor
Find the vertex and compare the opening, keeping the baseline fixed.
Start the interactive lesson →Open practice activity
What you will learn
- Measure angles with a protractor
Models used: number sentences, stories, diagrams.
Before you start
An angle is an opening
Look at the opening between the rays. Ignore how long the rays are.
Look at the opening between the rays. Ignore how long the rays are.
Longer sides do not make a wider opening. A square corner is a useful reference; a protractor measures from the correct baseline.
A small opening Open the rays wider. The angle gets larger.
A wider opening This opening is a square corner: a right angle.
A right angle A protractor counts the turn from the first ray. Start at the zero that lies on that ray.
60°
The second ray passes through sixty degrees
What did the picture help us see?
A protractor counts the turn from the first ray. Start at the zero that lies on that ray.
Worked example: A new example, together
Read the marked angle using the protractor.
Find the vertex and compare the opening, keeping the baseline fixed.
An angle measurement counts degrees from the correct baseline ray.
Use the zero scale aligned with the first ray. Centre the protractor at the vertex.
Use the zero scale aligned with the first ray. Read the marked angle using the protractor.
160 degrees
Centre the protractor at the vertex.
Use the zero scale aligned with the first ray. Read from zero on the first ray.
Use the zero scale aligned with the first ray. The second ray passes through 160°.
Use the zero scale aligned with the first ray. Check the result against the question we started with.
The supplementary angle is 20°; together they form 180°.
We found 160 degrees. An angle measurement counts degrees from the correct baseline ray.
Use the zero scale aligned with the first ray.
Common mistakes to discuss
- Reading the wrong scale. Start from the zero lying on one arm and read round from there. An angle just under a square corner reads 80°, not 100°.
Check your understanding
- Measure angles with a protractor
- recognise or measure a turn independently of ray length
- check the answer against the original quantities and units
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
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