Using a Protractor MathBitsNotebook.com Terms of Use   Contact Person: Donna Roberts
 A protractor is a tool used to measure angles in degrees.
 There are two basic types of protractors. One that measures 180º (half a circle). One that measures 360º (a full circle). We will be using the 180º type of protractor on this page.
 Take a Closer Look

Notice how the degrees are labeled on an arc on the protractor.
There are two sets of degrees going in opposite directions along an arc.
The top of the arc shows degrees from 0º to 180º from left to right.
The bottom of the arc shows degrees from 180º to 0º from left to right.
The smallest "tick marks" along the edge of the protractor represent 1º intervals.

While the 0º and 180º marks are not labeled on this protractor,
take note that they are represented by the bottom edge of the tool.

 Measuring an Angle

1. Place the "center point" (at the bottom of this protractor) on the vertex of the angle.

2. Line up one ray (side) of the angle with the zero degree mark of the protractor. On this protractor, the zero mark is the bottom edge of the protractor.

3. Read the degrees where the other ray of the angle crosses the protractor.

 When using a protractor to measure an angle, be sure your answer makes sense. Take a look at the angle to determine its approximate size.

The angle shown at the right is a 40º angle.
One ray of the angle lies on the bottom edge of the protractor (the zero tick mark at the right), and the angle OPENS up from that ray.

Notice the dashed blue arrow. It is following the bottom arc of the degrees, as that is the labeling that started with 0º. The answer is 40º, not 140º.

The degrees in this angle are being read on the top arc of the labeled degrees since we lined up one of the rays of the angle with the zero tick mark at the left side of the protractor.
Follow the red arrow to 75º.
When reading an obtuse angle (greater than 90º but less than 180º), remember to read FROM the 0º location on the starting ray at the zero tick mark.
Follow the red arrow to 115º

 Drawing an Angle
1. Start by drawing a straight line (a reference line) that will contain one of the rays of the angle. A horizontal line will be the easiest with which to work. Use a ruler or the edge of the protractor to draw this line. No freehand drawing.

2. Place a dot on the line to represent the vertex of the angle.

3. Place the "center point" of the protractor on the dot. Line up the reference line with the zero degree mark of the protractor.

4. Find the number of degrees you wish your angle to have. Read carefully along the degree arc. Place a dot at this location.

5. Draw the angle, connecting the dot to the vertex point. Use a straight edge to draw the final ray of the angle. No free hand drawing.

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