Measuring Angles with a Protractor

Key idea

Align the baseline and read the scale.

In depth

Place the centre of the protractor on the vertex and line one arm up with the zero line, then read where the other arm crosses the scale. Use the scale that starts at 0 on your baseline.

Worked example

If the baseline arm sits on the 0° mark of the inner scale and the other arm passes 50, the angle is 50°, not 130°.

Real-world uses

• Drawing a roof pitch for a model
• Checking a bike frame angle in school design work

Common mistake

Reading from the wrong scale. Check that the answer is acute or obtuse as expected.

How to learn this

1. Read the key idea and say it in your own words.
2. Work through the worked example, then try a similar problem.
3. Use the interactive lesson’s visual and mini-quiz to check the pattern.

Frequently asked questions

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Related beginner topics in Geometry

• Points, Lines and Planes – The undefined building blocks of geometry.
• Line Segments and Rays – A segment has two endpoints; a ray starts at a point and goes on forever.
• Angles – Two rays sharing an endpoint; measured in degrees.
• Types of Angles – Acute <90°, right 90°, obtuse between 90° and 180°, straight 180°.
• Complementary and Supplementary – Pairs summing to 90° or 180°.
• Triangles: Classification – By sides (equilateral, isosceles, scalene) and by angles.