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Step 1
Look at the ruler. The rulers on school supply lists are marked in inches and come in 12-inch lengths. Printers use 18 or 24-inch rulers, the traditional width of printing galleys, with measurements in inches and points. A 36-inch ruler is called a yard stick because it is three feet long. An architect's rule has three sides and has different scales on each side. Once you know what measuring system your ruler uses, you can begin to read it.
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Step 2
Take a plain old school ruler and look at it. It should be divided into twelve segments, numbered one through twelve. Each number will be next to a long line and there will be a group of lines between numbers. The second-longest line divides the inch into half-inches and each succeeding length into quarters, eighths and sixteenths of an inch. Note that there are eight sixteenths in a half inch, four sixteenths in a quarter and two sixteenths in an eighth.
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Step 3
Read your ruler by noting the inch, then adding the number of parts of the inch at the longest line you can find (say three quarters), then the next longest and so on. If, say, you end up with three quarters plus one eighth plus one sixteenth, you'll have to make your measurement in sixteenths and take twelve sixteenths plus two sixteenths plus one sixteenth to get fifteen sixteenths. Your total measurement would be one and fifteen sixteenths inches.
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Step 4
Read a metric ruler by first noting whether the measurement is in centimeters (cm) or millimeters (mm). A centimeter is equivalent to 0.39 inches. Most metric rulers are numbered in centimeters and marked to millimeters, ten mm to each cm. Since the metric system is based on the metric system, reading a metric ruler simply consists of noting the number, say two centimeters and counting the millimeters. The fifth millimeter is generally marked with a longer line than the others to give a reference point. If you read a three on the ruler above, then counted six millimeters toward the four cm mark, you would have 3.6cm or 36mm.
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Step 5
Look at other variations. Other types of rulers use units of measurement that designers, scientists or trades people need to do their jobs. The architect's ruler above has three sides with twelve different "scales"--a rule with distances marked in different lengths to make accurate drawings of objects too large to draw life-size. With a little practice, these rulers can be used for things like designing a tree house or doing school projects.
