Millimeter to Inch Converter

The Math

One inch equals exactly 25.4 millimeters by international agreement, so the tool divides your millimeter value by 25.4. Enter 100 mm and you get 3.937 inches; enter 10 mm and you get 0.394 inches.

Why Three Decimal Places

The result is rounded to three decimals rather than two, because millimeter-scale measurements — screw threads, PCB tolerances, drill bit sizes — often need more precision than a whole-number-of-inches figure would give. The difference between 0.393" and 0.394" can matter when a part needs to physically fit.

Reference Sizes

A4 paper is 210 mm wide = 8.268 inches. A credit card is about 85.6 mm long = 3.370 inches. An M3 screw thread (3 mm nominal) converts to 0.118 inches — useful when cross-referencing metric hardware against an imperial drill index.