Math Help: Using a Protractor

Protractors in Math

Geometry Measurement

In elementary school geometry, you'll work with angles. An angle occurs when two lines intersect (cross), and it's measured by drawing an imaginary circle that has the point of intersection as its center. Every circle is divided into 360 degrees, and the number of degrees that separate the two lines are the angle's measure.

If you draw a straight line through a circle's center, the measure on either side will be 180 degrees (half of 360). If you draw a perpendicular line to make a cross in the middle of the circle, this will create four identical right angles, which are each 90 degrees (half of 180). If an angle is less than 90 degrees, it's called an acute angle. If it's more than 90 degrees, it's an obtuse angle.

Using a Protractor

A protractor is shaped like a half moon, and the rounded part has degrees up to 180 listed around it. There will be two sets of lists going in opposite directions, so be careful to use the right one.

Measuring an Angle

If you're using a protractor to measure an angle that's already drawn, line up the bottom part of the protractor with one side of the angle (it doesn't matter which one). Be sure that the point where the two lines intersect is in the middle of the little circle at the bottom of the protractor.

Next, starting at zero, follow the curve of the protractor up until you reach the second line. If your lines are too short to reach the curve of the protractor, extend them using a ruler. The point where the line intersects with the protractor tells you the angle measure.

Double-check your answer to be sure it makes sense. If your angle is clearly less than 90 degrees, but your answer is greater than 90 degrees, this means you used the wrong side of the protractor. Measure the angle again, and be sure to start at zero degrees, not 180 degrees.

Drawing an Angle

You may also use a protractor to draw an angle. Begin by drawing a straight line using a ruler, and then line your protractor up with this line. One end of the line should be at the center of the circle drawn on the protractor.

Starting from zero, move up the curve of the protractor until you reach the desired number of degrees, and make a mark next to that number. Then, remove the protractor and use your ruler to draw a straight line connecting that dot and the end of the other line.