Working with the metric system sometimes means converting measurements using simple mathematical formulas. Because metric measurements are expressed in units of 10, it is possible to quickly grasp the increasing difference between millimeters, centimeters and meters. Converting a familiar measurement such as inches into millimeters takes only seconds when using the correct multiplication factor.
Measure the distance to be converted from inches, using a ruler or tape measure. For example, a CD jewel case is 5 inches wide and 5 inches long.
Multiply the number of inches by 25.4 to convert to millimeters.
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Use the example to understand the conversion: A CD jewel case 5 inches wide times 25.4 equals 127 mm. As an equation, it is expressed like this: 5 x 25.4 = 127.
TL;DR (Too Long; Didn't Read)
Divide the total millimeters by 25.4 to convert the measurement back to inches.