A protractor is a tool used to measure angles. Most protractors measure angles in degrees (°).

When using a protractor, notice that the outside set of numbers goes from 0 to 180 degrees where the 0 is on the left side of the protractor while the inner set goes from 180 to 0 degrees where 0 is on the right side of the protractor. Whichever side you line up with the zero-degree line determines which set of numbers to use.

How to measure angles using a protractor

Let's measure ∠STV below.

So, ∠STV has a measure of 50°.

Similarly, you can place TV so the vertex is inside the hole of the protractor and lines up with the 0 line to the left side of the protractor. Then, read the degree measure on the outside set of numbers where side TS intersects the edge of the protractor. It also shows ∠STV has a measure of 50°.

How to draw angles with a protractor

Protractors can also be used to draw an angle having a specific measure. For example, to draw ∠OMP having a measure of 30°:

We could also construct ∠OMP by placing OM on the 0 degree line on the right side of the protractor as long as vertex M is centered at the midpoint of the protractor. Similarly, we could have constructed OM on the 0-degree line first then placed point O on the 30 degree measure to draw OM.