How to Read a CM Ruler


There are two rulers used in elementary schools. The first is an English ruler, which is difficult to learn because it involves fractions. The second is a metric ruler, which is used to measure smaller items than an English ruler. It uses centimeters and millimeters.

Pencil and ruler
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Things You'll Need

  • Small objects
Step 1

Examine your ruler. You will see two sets of lines. The first set of lines are longer, farther apart and numbered. These are called centimeters, or cm for short. The second set of lines are thinner and closer together. They fall in between the larger numbers. These are called millimeters or mm for short. They are not numbered. There are 10 millimeters in each centimeter.

Cm ruler
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Step 2

Pick a small object to measure. This can be a pencil, paper clip, penny or refrigerator magnet. Whatever you have on hand will do.

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Step 3

Place the end of your object, such as the eraser on a pencil, against the beginning edge of the ruler. Make sure they line up perfectly.

Pencil marking ruler
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Step 4

Place your finger on the mark where the object ends, then remove the object.

Ruler next to pencil and eraser
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Step 5

Look for the last centimeter mark that your object passed. It is possible that the object landed directly on a numbered centimeter mark. For example, if your pencil ends on the 5 centimeter mark, then it is 5 centimeters long. If, however, it went slightly past the 5 centimeter mark, write down 5 centimeters and continue to the next step.

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Step 6

Count the little lines after the last centimeter mark your object passed until you land on the line that matches the end of the object. So if your pencil ends three lines after the 5 centimeter mark, it would be 5 centimeters and 3 millimeters long.

Ruler on graph paper
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Tips & Warnings

  • Because each millimeter is 1/10 of a centimeter, you can write your measurement as a decimal. Place the number of millimeters after the decimal point; e.g., the pencil would be 5.3 cm.
  • If you have a large object to measure, you will want to use the English ruler.


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