The millimeter (mm) is the smallest commonly used metric unit of length, equal to one-thousandth of a meter. It is widely used in engineering drawings, manufacturing tolerances, and rainfall measurements where precision matters. One millimeter equals roughly 0.0394 inches, making it ideal for describing thin materials like paper or wire.

The inch (in) is an imperial unit of length equal to exactly 25.4 millimeters, widely used in the United States, Canada, and the United Kingdom. It is the standard measurement for screen sizes, pipe diameters, and mechanical fasteners, and remains common in everyday use across North America. One inch equals exactly 2.54 centimeters.