How to Measure an Angle With a Protractor

Common protractor

Computer technology can automate many tasks, but the simplicity and ease of a protractor make it a useful tool. You can use a protractor to measure and draw angles. Protractors are commonly made of translucent plastic in a semicircular shape. The unit of measure on the arc is a degree, and a half-circle protractor has 180 degrees marked on it to measure angles.

Instructions

1. Place the protractor on top of the angle you want to measure.

2. Find the center mark of the protractor along its straight edge, or baseline.

3. Place the center mark on top of the vertex of the angle, or the point where the two lines of the angle meet.

4. Rotate the protractor while keeping the center mark and the vertex aligned until one line of the angle matches up with the baseline of the protractor. The other line of the angle should cross one of the degree measurements on the semicircle of the protractor.

5. Look at the angle you are measuring to see if it's greater or less than 90 degrees. A 90-degree angle looks like an L. If the angle you are measuring is wider than the angle in an L, it is greater than 90 degrees. If the angle you are measuring is tighter than the angle in an L, it is less than 90 degrees.

6. Read the numbers on the protractor where the line of your angle crosses them. The protractor has two sets of numbers. If you determined your angle is greater than 90, choose the number that is larger than 90. If your angle is less than 90 degrees, choose the number that is less than 90. This number is the degree measurement of your angle.

Tips & Warnings

• If the lines of the angle you're trying to measure are not large enough to reach the degree marks on your protractor, use a ruler to extend a line until it reaches the arc of the protractor.

References

• "Architectural Graphics"; Frank Ching; 2009
• "Homework Helpers: Geometry"; Carolyn C. Wheater; 2007