To convert length x width dimensions from inches to millimeters we should multiply each amount by the conversion factor. One inch equals 25.4 millimeters, in order to convert 3 x 21 inches to millimeters we have to multiply each amount of inches by 25.4 to obtain the length and width in millimeters. In this case to convert 3 x 21 inches into millimeters we should multiply the length which is 3 by 25.4 and the width which is 21 by 25.4. The result is the following:
3 x 21 inches = 76.2 x 533.4 millimeters
An inch (symbol: in) is a unit of length. It is defined as 1⁄12 of a foot, also is 1⁄36 of a yard. Though traditional standards for the exact length of an inch have varied, it is equal to exactly 25.4 mm. The inch is a popularly used customary unit of length in the United States, Canada, and the United Kingdom.
The millimeter (symbol: mm) is a unit of length in the metric system, equal to 1/1000 meter (or 1E-3 meter), which is also an engineering standard unit. 1 inch = 25.4 mm.
To calculate a inch value to the corresponding value in millimeters, just multiply the quantity in inches by 25.4 (the conversion factor).
millimeters = inches * 25.4
The factor 25.4 is the result from the division 1 / 0.0393701 (millimeter definition). Therefore, another way would be:
millimeters = inches / 0.0393701
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