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.

Direct Rates
1 millimeters = 0.0393700787 inch
1 inch = 25.4 millimeters

Using the Factor-Label Method of Conversion
So, to convert millimeters to inches, method known as the factor-label method of conversion must be done.
1.Firstly, we know that 25.4 x 10-3 meters equals 1 inch. But before that, meters must be converted to millimeters. 1 meter = 1,000 millimeters
2.Considering how 25.4 x 10-3 meters equals an inch, and 1,000 millimeters equals a meter, we can find the relationship between an inch and a millimeter by finding the product of: 25.4 x 10-3 x 1,000
3.This equals to 25.4 millimeters equaling an inch. This basically means that 1 millimeter equals 1/25.4 of an inch.

Work Shown
1 mm / 1 x 10-3 m = 1 m / 100 cm = 1 cm / (1/2.54) in.
Crossing out the meters (m) and centimeters (cm) gives us a relationship between the millimeter (mm) and the inch (in.).