A ruler is used in math, drawing, carpentry or any other application where it is important to know the length of something. Within the United States, the measurement unit is in inches, where 12 inches equals one foot and three feet equal one yard. A knowledge of fractions is helpful in understanding this type of measuring system. These steps will familiarise you with reading a ruler and brush up your math skills at the same time.
Look at the side of the ruler labelled "INCHES." For every inch, you will see a number clearly marked next to a line that is the longest. Within the marked inches will be lines in decreasing lengths. The second-longest measures 1/2 inch, the next measures 1/4 inch, then 1/8 inch, and the shortest measures 1/16 inch.
Using the diagram, you can begin to understand the logic of each mark. Looking at the 1/2-inch mark, you will see that it is the second longest. It is halfway between each inch mark. Halfway between each 1/2-inch mark is the third longest line, 1/4 inch. Using fractions, you can count from the 1/2-inch mark and add 1/4 inch to get 3/4: 1/4 + 1/4 + 1/4 = 3/4. (1/2 = 1/4 + 1/4 = 2/4).
From the 1/4 line, you will see that the next shortest line is in between it. It is 1/8. It can be helpful to think that 1/8 + 1/8 = 2/8 (2 divided by 2 = 1, 8 divided by 2 = 4, so it creates 1/4). This also is the logic for the next smallest line that is 1/16.
When reading a ruler for the first time, it can be helpful to count each mark, typically measuring 1/16 of an inch. This will also help with understanding fractions as well. Example: You count eight marks, equalling 8/16 of an inch or 1/2 inch. (8 divided by 8 = 1, 16 divided by 8 = 2, so it simplifies to 1/2). You will notice that it will land on a mark that is longer.
Continue to practice counting each mark. As you become more familiar with each line length, it will be easier for you to determine the measurement without having to always count them out. Your knowledge of fractions also will improve, and you will feel more comfortable as time goes on.
Some rulers vary, and may have marks that are 1/32-inch apart. It will be noticeable that they are closer together and even shorter.