One foot equals 304.8 mm, to convert 21 feet to mm we have to multiply the amount of feet by 304.8 to obtain the width, height or length in mm. 21 feet are equal to 21 x 304.8 = 6400.8 mm.
A foot (symbol: ft) is a unit of length. It is equal to 0.3048 m, and used in the imperial system of units and United States customary units. The unit of foot derived from the human foot. It is subdivided into 12 inches.
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 foot value to the corresponding value in millimeters, just multiply the quantity in feet by 304.8 (the conversion factor).
millimeters = feet * 304.8
The factor 304.8 is the result from the division 1 / 0.003281 (millimeter definition). Therefore, another way would be:
millimeters = feet / 0.003281
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