There are 25.4 millimeters in 1 inch. An inch is the name of a unit of length in a number of different systems, including Imperial units and U.S. customary units.
A millimeter is one-thousandth of a meter, which is the International System of Units base unit for length. According to Metric-Conversions.org, millimeters are used as a standard measure of length in engineering and commercial applications where accuracy greater than the closest centimeter is required. A formula for converting millimeters to inches is: millimeters = inches / 0.03937. For example, the number of millimeters in 1 foot is given by 12 (the number of inches in 1 foot) divided by 0.03937, which yields 304.8 millimeters. The head of a pin head measures approximately 2 millimeters in diameter, and a compact disc is approximately 1.2 millimeters thick.