Inches is a measure from the imperial system used in the USA, Great Britain and some other countries. Millimeters are one tenth of a centimeter from the metric system. To convert from one value to another, you can remember that one inch contains 25.4 millimeters. Or, you can skip remembering it and use our quick and handy online inches to mm calculator. It is also possible to add it to your browser bookmarks to always have it at hand.