How to Read Protractors

Protractor with arm

A protractor is a common instrument used to measure angles. You may use one in a math class for homework or to measure angles and corners when building. There are many different types of protractors. The easiest one to use and read is a half-circle that measures 180 degrees. Once you know how to read a protractor, you will be able to determine the number of degrees in different angles.

Things You'll Need

• Protractor

Instructions

Reading a Protractor

1. Identify the parts of the protractor. The vertex point is in the lower, center part of the protractor. The zero line goes across the bottom of the protractor. The two sets of numbers are the inner scale and the outer scale.

2. Look at the numbers on the protractor. The outer scale starts on the left and goes from zero to 180 clockwise. The inner scale starts on the right and goes from zero to 180 counterclockwise.

3. Determine if you are measuring an acute or obtuse angle. An acute angle is less than 90 degrees, an obtuse angle is greater than 90 degrees.

4. Determine if the arm of the angle passes through zero on the inner or outer scale. You will use the corresponding numbers to measure an angle. (An angle is composed of two lines; each of those lines is called an arm.)

5. Read the number on the scale where the second arm passes through. That is the measure, in degrees, of the angle.

Tips & Warnings

• Most common protractors measure 180 degrees. A straight line measures 180 degrees. A circle measures 360 degrees.

Resources

• Math Open Reference, Animation on Using a Protractor