| Millimeters (mm) | Feet (ft) |
|---|---|
| 1 Millimeter | 0.00328083989501 ft |
| 2 Millimeters | 0.00656167979003 ft |
| 3 Millimeters | 0.00984251968504 ft |
| 4 Millimeters | 0.0131233595801 ft |
| 5 Millimeters | 0.0164041994751 ft |
| 10 Millimeters | 0.0328083989501 ft |
| 20 Millimeters | 0.0656167979003 ft |
| 25 Millimeters | 0.0820209973753 ft |
| 50 Millimeters | 0.164041994751 ft |
| 100 Millimeters | 0.328083989501 ft |
| Reference | Millimeters (mm) | Feet (ft) |
|---|---|---|
| A sheet of A4 paper (long side) | 297 mm | 0.974409 ft |
| Average adult human height | 1700 mm | 5.57743 ft |
| A football pitch (length) | 105000 mm | 344.488 ft |
| A marathon | 42195000 mm | 138435 ft |
| Height of Mount Everest | 8849000 mm | 29032.2 ft |
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.
The foot is a unit of length equal to exactly 0.3048 metres, or twelve inches. Its plural is feet and its symbol is ft, though a single prime is used in technical drawing and a straight apostrophe informally.
Units based on the human foot appear across the ancient world, and their lengths varied considerably. The Roman pes measured about 296 millimetres and was divided into twelve unciae, an arrangement English inherited directly. Medieval Europe supported dozens of local feet, and a merchant crossing a few borders might encounter several. The English foot was standardised in stages, but its exact modern value dates only from the International Yard and Pound Agreement of 1959.
Aviation is the foot's most significant surviving international domain. Aircraft altitude is reported in feet almost everywhere, and flight levels are expressed in hundreds of feet, so FL350 means 35,000 feet. Russia and China historically used metres, and China's transition to feet for most flight levels in 2011 removed a genuine safety hazard at the boundaries of adjoining airspace. Vertical separation standards are defined in feet, and reduced vertical separation minima allow 1000 feet between aircraft above 29,000 feet.
Construction in the United States works in feet and inches throughout, and lumber, ceiling heights and room dimensions all follow. Broadcasting and film retain the foot for lens focus scales and for measuring film stock. Water depth in diving is given in feet in American practice and metres elsewhere, a divergence that dive computers accommodate by offering both.
Twelve as a divisor is the foot's practical strength. It divides evenly by two, three, four and six, which suits carpentry and layout work where thirds and quarters are common. A decimal unit divides cleanly only by two and five.
Water and timber keep two of its derivatives alive. The fathom, six feet, measured the depth of water for as long as a sounding line was thrown by hand, and old charts are still marked in fathoms even where new ones use metres. The board foot, a volume of one foot square by one inch thick, remains the trading unit of sawn timber in North America, so a lumber yard quotes prices per thousand board feet. Both illustrate a general habit of the imperial system: rather than adding prefixes, it names each new quantity after the job it does, which makes the units memorable to those who use them and opaque to everyone else.
One foot equals 12 inches, 0.3048 metres, or 30.48 centimetres exactly.