A millimeter is a common unit of length within the metric system. This system of measurements is commonly used throughout the world, but in the United States, the U.S. customary system of measurements is employed more frequently. The inch is the basic unit of length within the U.S. system. If you have a ruler that is marked in inches, you can still measure millimeters as long as you know how to convert between the two units.
Instructions
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1
Place the end of your ruler at the beginning point of the object that you are measuring. Lay the ruler flat against the object.
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2
Stand directly over the ruler to read its measurement in inches. Write this measurement down.
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3
Multiply the length in inches times 25.4, since there are 25.4 mm in every inch. For example, if the length in inches was 10, you would multiply 10 times 25.4 to get a length of 254 mm.