MAT1793-ABED_1: Using a Protractor

Constructing Knowledge

A protractor is a tool used to measure angles. Angles are measured in degrees. Geometry sets can include full circle (360°) protractors, however semi-circle (180°) protractors are much more common. The main parts of a protractor are:

Proper placement of the protractor is the first step to measuring an angle. Place the origin of the protractor where the two rays (arms) of the angle meet (vertex), and have the baseline covering one of the arms.

Protractors have two scales, the inner and outer scales. Errors can be made if you take the reading from the wrong scale. Read from the scale that starts at '0' at the arm of the angle that is covered by the base line of the protractor.

If the initial arm starts on the left, use the outer scale. If the initial arm starts on the right, use the inner scale.

Inner Scale

Outer Scale

Both of the angles measure 55°. It is an important skill in this chapter to be able to correctly use a protractor.

Multimedia

A video is provided describing how to use a protractor.