Learn about the protractor. The straight edge on the bottom of this semicircular instrument is marked off as a 6-inch ruler. The smaller straight line on the inside is marked in centimeters. Along the outside semicircle are the degrees in two layers. Reading from left to right, the degrees go from 0 degrees to 180 degrees. The inside markings read from right to left, again going from 0 degrees to 180 degrees. In the center of the straight edge is a small circle (also called the origin) used for correctly aligning the protractor with the vertex of the angles.
Place the protractor at the baseline of the angle. Line up the baseline on the protractor straight edge with the bottom line of the angle. Place the circle on the protractor on the vertex of the angle. Think of it as aligning the cross hairs.
Read off the degrees from the set of numbers that has 0 degrees falling on the line of the angle. If the line of the angle is to the right of the origin point on the protractor, read from the set of numbers that start with 0 degrees on the right and increase to the left. Read the degree where the other line of the angle crosses the protractor. For example, the second line may cross the protractor at the number 45. Then it is a 45-degree angle.
Measure the angles. If the angle is a straight line perpendicular to the base, then it is a right angle, which is 90 degrees. It is the only angle that will read correctly no matter which side of the protractor you start from.
Check that the protractor is properly placed on the baseline of the angle and that the second line is visible. If the lines are short, extend the line so it can be seen at the semicircular edge of the instrument. Measure the angle. An acute angle is less than 90 degrees. A good memory aid is to picture something cute and small.
Determine the angle you have to measure. If it is more than 90 degrees, then it is an obtuse angle. Some protractors have only one set of markings, going from 0 degrees to 180 degrees. Take the reading and subtract the number from 180 degrees to get the correct angle. For example, if the angle reads 35 on the single scale protractor, subtract 35 from 180 to get the correct reading of 145 degrees.