To convert a unit from millimeters to inches, multiple steps are necessary as the two units are in a different system of units. Millimeters is in a number form, 1 x 10-3 meters (the standard unit of length), and is one of the metric unit systems and a part of the International System of Units (SI). On the other hand, an inch is a unit of length in a few systems such as in the English units, Imperial units, and the United States customary units. It is basically 1/36th of a yard or 1/12th of a foot.