Standard rulers usually measure in two different units of length, inches on one side and centimeters on the other side. Centimeters are used by the International System of Units, which is based on the factor of 10. There are exactly one hundred centimeters in one meter, and exactly 10 mm, or millimeters, in 1 cm. Millimeters can be easily seen on a standard centimeter ruler as the small markings between the centimeter intervals. The 5 mm marking between centimeter marks is often longer than the other millimeter marks.
Locate the centimeter side of your ruler. It will usually be marked with the abbreviation "cm" for centimeter.
Look closely at the small intervals between the "0" and "1" cm marks. Each interval represents 1 mm. Some rulers have the abbreviation "mm" for millimeter at the start of the centimeter side.
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Count 1 mm for each small marking between the centimeter marks. If you are measuring a small object, place one edge of the object at the "0" mark and count how many small markings the object covers. Each marking is 1 mm, so if the object covers six markings from edge to edge, it measures 6 mm.