Conversion from Millimeters to Inches

Information about the Millimeter (mm)

The millimetre is one thousandth of a metre, written mm. It is the smallest division marked on most rulers and tape measures, which makes it the practical floor of everyday measurement in countries using the metric system.

Technical drawing has settled on the millimetre almost universally. Mechanical engineering drawings state dimensions in millimetres by default, usually without writing the unit at all, because the convention is understood. This avoids the decimal points that centimetres would introduce and the large numbers that micrometres would require. A component 45.5 mm long is easier to read and harder to misinterpret than one 4.55 cm or 45500 µm long.

Meteorology uses the millimetre for rainfall. A reading of 25 mm means that rain would stand 25 millimetres deep on a flat surface that did not drain, which corresponds to 25 litres per square metre. Snowfall is usually reported as depth in centimetres and separately as liquid water equivalent in millimetres.

Medicine relies on the unit for tumour dimensions, wound measurement and the size of anatomical structures on imaging. Ophthalmic prescriptions specify pupillary distance in millimetres. Blood pressure retains the older unit of millimetres of mercury, a pressure measurement whose name preserves the height of a mercury column.

Paper, sheet metal and glass are specified by thickness in millimetres. A standard sheet of office paper is about 0.1 mm thick. Domestic window glass is commonly 4 mm. Firearm and ammunition calibres are frequently given in millimetres, as in 9 mm.

Fastener sizes follow the unit closely. Metric bolts and screws are designated by their nominal diameter in millimetres, so an M8 bolt has an eight-millimetre thread. Spanner and socket sizes are marked the same way, measuring across the flats of the fastener head. This is why a metric toolkit contains an unbroken run of sizes rather than the fractional inch steps of an imperial set.

Medicine measures pressure with it as well as distance. Blood pressure is reported in millimetres of mercury, the height of a mercury column the pressure would support, so a healthy reading of 120 over 80 refers to two lengths rather than to any force directly. The convention dates from the mercury manometer and has survived every instrument that replaced it, because the numbers are familiar to every clinician alive. The same unit measures intraocular pressure in the eye, central venous pressure and the partial pressures of gases in blood, and it appears in vacuum work as the torr, which is one millimetre of mercury under another name.

One millimetre equals 1000 micrometres, 0.1 centimetres, or 0.001 metres. It is roughly 0.03937 inches, so 25.4 millimetres make exactly one inch.

Information about the Inch (in)

The inch is a unit of length equal to exactly 25.4 millimetres. It is one twelfth of a foot and one thirty-sixth of a yard. The symbol is in, though a double prime is common in technical contexts and a straight double quote is often substituted informally.

The name comes from the Latin uncia, meaning a twelfth part, which also gave English the word ounce. Early definitions were anatomical or agricultural. A statute of Edward II defined the inch as three barleycorns laid end to end, and the width of a man's thumb served as a working approximation in several European languages. Dutch and Swedish still use words for inch that mean thumb.

Its modern precision dates from 1959, when the International Yard and Pound Agreement fixed the yard at exactly 0.9144 metres across the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. Before that agreement the American and British inches differed very slightly, which mattered in precision manufacturing. The United States retained a separate survey inch for land measurement until it was formally retired at the end of 2022.

The inch remains dominant in several international industries regardless of local measurement systems. Screen sizes are quoted in inches worldwide, measured diagonally. Tyre rim diameters, bicycle wheel sizes, plumbing pipe bores and rack-mounted equipment all use inch designations, and the standard 19-inch rack is a global fixture. Photographic sensor sizes use inch fractions that no longer correspond to any real dimension, a survival from television camera tube measurements.

Subdivision is traditionally binary rather than decimal. An inch is halved repeatedly into eighths, sixteenths and thirty-seconds, which suits ruler markings and fits the fractional sizing of drill bits and spanners. Decimal inches are used in machining, where thousandths of an inch are called thou or mils.

Screens and printing keep it in daily use far outside the countries that use it for anything else. Televisions, monitors and phones are sold by the diagonal in inches everywhere, so a 55-inch television has the same name in Paris as in Chicago. Typography rests on the same base: there are 72 points to the inch, a convention inherited from the printing trade and written into every computer font system, and image resolution is quoted in dots or pixels per inch. Anyone laying out a page in a country that has been metric for a century is therefore working, without particular thought, in twelfths and seventy-seconds of an inch.

One inch equals 25.4 millimetres, 2.54 centimetres, or 0.0254 metres exactly.