📐 How to Read a Protractor

A protractor measures angles in degrees. But watch out: it has two rows of numbers! Let’s learn together which one to read.

Along the bottom of a protractor there is a straight baseline, with a centre point in the middle. The outer scale counts from the 0 on the right, the inner scale from the 0 on the left, up to 180.

1. Put the vertex of the angle on the centre point. 2. Line one arm up with the 0 line. 3. The blue arm is on the 0 at the right, so count on the outer scale: 10, 20, 30, 40, 50. The angle is 50°!

The most common mistake: reading the wrong scale! Here the blue arm is on the 0 at the left, so the inner scale starts at 0. The angle is 50°, not 130°.

Before you measure, estimate: is the angle bigger or smaller than a right angle (90°)? This one is bigger than 90°, so it can’t be 60°. It is 120°. Estimating protects you from reading the wrong scale.

An angle smaller than 90° is an acute angle. Exactly 90° is a right angle. Between 90° and 180° is an obtuse angle. Exactly 180° is a straight angle.

Adjacent angles next to each other on a straight line add up to 180°: 50° + 130° = 180°. The two numbers on a protractor always add up to 180 too!

When two straight lines cross, the angles facing each other are called vertically opposite angles. They are equal: 50° and 50°, 130° and 130°.

Summary: vertex on the centre, one arm on 0, then read the scale that starts at that 0. Estimate first! Adjacent angles on a straight line make 180°, and vertically opposite angles are equal. Now try it in Angle Hunter!