How to Interpret a Ruler

Metric Ruler

• 1

  Count the number of marks between the numbers, remembering that the numbers on a metric ruler represent centimeters. Note that there are 10 millimeter marks between each centimeter mark.

• 2

  Look for the slightly longer millimeter mark in the center of the millimeter marks. This represents the halfway point between the centimeter preceding and the centimeter following.

• 3

  Read the metric ruler by looking for the preceding centimeter, counting the millimeters and using the millimeter as a decimal. For example, a mark four spots after 3 would read 3 centimeters, 4 millimeters---or 3.4 centimeters.

English Ruler

• 4

  Count the number of marks between each number, remembering that the numbers represent inches and the smaller marks are fractions of those inches. Note that English rulers may have eight marks or 16 marks between each inch, but the latter is more common.

• 5

  Look for the three longest marks between two inches. These represent the measurements of fourths: 1/4, 2/4 (or 1/2) and 3/4. Look for the next smallest marks, which represent measurements of eighths: 1/8, 3/8, 5/8 and 7/8. Look for the smallest eight marks, which represent measurements of sixteenths: 1/16, 3/16, 5/16, 7/16, 9/16, 11/16, 13/16 and 15/16.

• 6

  Read an English ruler by looking for the whole number preceding the measurement, and then counting the fractional tick marks, identifying what that measurement is by the sizes specified in Step 2. Leave the answer in fractional form or use a calculator to convert the number to decimal form. For example, a measurement past the 3-inch line and on the third smallest line would be 3 and 5/16 inches, which also equals 3.3125.