How to Find Angles Using a Protractor

A protractor is an instrument used to determine the size of an angle. Protractors are usually first used as early as upper elementary or middle school. Although protractors come in a variety of shapes and sizes, the most common in education is the semicircle protractor, which measures angles from 0 to 180 degrees. Some semicircle protractors have a dual scale so angles can be measured easily whether they open to the right or to the left.

Instructions

1. Place the origin of the protractor on the vertex of the angle to be measured. The origin is usually a small hole or crosshair located near the bottom of a semicircle or rectangular protractor or near the center of a square or full-circle protractor.

2. Align the baseline of the protractor with one of the lines that forms the angle while keeping the origin on the vertex. The baseline connects the 0 degree mark with the origin and will usually be imprinted on the protractor.

3. Read the number on the scale where the other line intersects. If you are using a dual-scale protractor, make sure you are reading the correct scale. One scale, usually the outer scale, will measure from right to left, while the other scale, usually the inner scale, will measure from left to right. The scale containing the 0 that was used for your baseline will be correct.