Just as larger measurements have a multitude of different units to gauge them, so do smaller measurements. The millimeter and thousandth of an inch are two minute units of length and distance. The millimeter is a minuscule measurement based on the meter. The thousandth of an inch, also known as a thou or a mil, is employed in arenas where exact precision is important. The millimeter is like the thousandth of an inch in that it is also a thousandth of a meter. You can convert millimeter measurements to thousandths-of-an-inch equivalents by conversion factors for inches, modified for a smaller size.

Multiply the measurement in millimeters by 39.3700787402 to convert the measurement to thousandths of an inch. For an example, 500 mm multiplied by 39.3700787402 converts to 19685.0394 thousandths of an inch.

Divide the millimeter measurement by 0.0254 to also convert to a measurement in thousandths of an inch. As an example, the same length of 500 mm divided by 0.0254 also converts to 19685.0394 thousandths of an inch.

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Convert the millimeter measurement to its equivalent in thousandths of an inch with a conversion website like Convert Units. Enter the number of millimeters where prompted, and then click the "Convert!" button. The conversion will appear underneath the original amount.