Article — mm to inches, cm, m, ft Converter
Millimeters to inches, cm, feet, and meters: the conversion guide
A millimeter (mm) is one-thousandth of a meter, the SI base unit of length. To convert millimeters to inches, divide by 25.4, the exact factor set by the 1959 International Yard and Pound Agreement. So 25.4 mm = 1 in, 100 mm = 3.937 in, and 1000 mm = 39.37 in (about 3 ft 3.4 in). The metric chain extends both ways: 10 mm = 1 cm, 1000 mm = 1 m, 1,000,000 mm = 1 km.
Millimeters appear everywhere there is a need for sub-centimeter precision: machining tolerances, electronic component sizes, medical implants, dental restoration, photography lens diameters. The unit lives in the gap between casual measurement (centimeters) and precision work (micrometers). It is small enough to be useful, large enough to read with a ruler.
What is a millimeter?
A millimeter is 0.001 meter. The "milli" prefix means one-thousandth, the same prefix used in milligrams (0.001 g) and milliliters (0.001 L). Its symbol is mm. The British spelling is millimetre. In SI base units, 1 mm equals 10^-3 m. The unit fits naturally into the decimal metric system, scaling by powers of ten in either direction.
The meter itself is defined by the speed of light: light travels exactly 299,792,458 meters per second, and one meter is the distance light covers in 1/299,792,458 of a second. This definition has been the international standard since 1983, replacing the older platinum-iridium meter bar in Paris. The BIPM in Paris maintains the SI brochure that codifies the definition.
The original meter was defined in 1791 as one ten-millionth of the distance from the equator to the North Pole through Paris. Surveys turned out to be slightly off; the actual distance is closer to 10,001,966 meters. The error was small enough that the platinum bar made to match the survey survived as the standard for 90 years. NIST keeps a copy in Gaithersburg, Maryland.
Millimeters to inches conversion
To convert millimeters to inches, divide by 25.4. So 50 mm equals 1.9685 in, 100 mm equals 3.937 in, and 250 mm equals 9.842 in. The factor is exact, not a measurement. In 1959 the US, UK, Canada, Australia, New Zealand, and South Africa signed the International Yard and Pound Agreement, which set the yard at exactly 0.9144 m. Since a yard contains 36 inches, the inch became 25.4 mm exactly.
1 in = 25.4 mm 1 ft = 304.8 mm1 cm = 10 mm 1 m = 1000 mmThe reverse conversion multiplies inches by 25.4. So 3/8 in equals 9.525 mm, and 1/2 in equals 12.7 mm. US hardware comes in fractional inch sizes (1/16, 1/8, 3/16, 1/4...) while European hardware uses round millimeter sizes (M3, M4, M6, M8). They look similar but rarely interchange: an M6 bolt is 6.0 mm, while a 1/4 in bolt is 6.35 mm; thread pitch and angle differ even when diameters are close.
Millimeters to centimeters and meters
Millimeter conversions inside the metric system are decimal shifts. To go from millimeters to centimeters, divide by 10. To go from millimeters to meters, divide by 1000. To go from millimeters to kilometers, divide by 1,000,000. There are no awkward factors, no rounding error. Each step is a power of ten.
Common values: 100 mm = 10 cm = 0.1 m. 500 mm = 50 cm = 0.5 m. 1,500 mm = 150 cm = 1.5 m, about chin height for a 5-foot adult. The smaller end of the scale goes the other way: 0.1 mm = 100 micrometers, the typical thickness of a sheet of office paper. 0.001 mm = 1 micrometer, used in semiconductor manufacturing.
Millimeters to feet and yards
To convert millimeters to feet, divide by 304.8 (exactly 12 times 25.4). So 1000 mm equals 3.281 ft, and 5,000 mm equals 16.404 ft. Yards add another factor of 3: divide millimeters by 914.4 to get yards. The factor 304.8 mm = 1 ft has been exact since 1959.
Construction picks the unit by region. North American framing uses 2 by 4 lumber at 38 mm by 89 mm nominal, dimensioned in inches but shipped in millimeter-stamped Canadian factories. European framing uses 38 x 89 mm or 45 x 95 mm in metric labels. Plumbing follows the same split: nominal 1/2 in pipe in the US is the same fitting as DN15 (15 mm) in Europe, but they are not directly compatible.
Millimeter reference chart
- 0.1 mm = sheet of office paper.
- 0.76 mm = a credit card.
- 1 mm = a stack of 10 sheets, or about a grain of sand.
- 1.6 mm = standard PCB (printed circuit board).
- 2.54 mm = exactly 0.1 in, the classic IC pin pitch.
- 6.35 mm = 1/4 in (audio jack, drill bit, US wrench).
- 10 mm = smartphone thickness, 1 cm.
- 25.4 mm = exactly 1 inch.
- 304.8 mm = exactly 1 foot, common ceiling tile width.
- 1000 mm = 1 meter, hip/waist height for an average adult.
Common millimeter conversion mistakes
The most common error is the decimal shift between mm and cm. 10 mm equals 1 cm, not 10 cm. A 100 mm part is 10 cm, not 100 cm; a 100 cm part is 1,000 mm. Get the wrong direction and the part is ten times the wrong size. Hardware specs that mix mm and cm in the same document are a frequent source of returns.
The classic typo. 2.54 is the number of centimeters per inch; 25.4 is the number of millimeters per inch. Dropping the zero gives a part that is one-tenth the size needed. Anyone who has ordered hardware in two unit systems has seen this go wrong at least once.
A second error is mixing fractional inches with decimals. 3/8 inch is exactly 0.375 inch, which converts to 9.525 mm. Writing it as 0.38 in is a rounding error of 0.005 in (0.127 mm). For precision machining tolerances of plus or minus 0.025 mm, that drift matters.
A US tape measure marked in 1/16 in has 16 ticks per inch, each tick worth 1.588 mm. So if you need 10 mm, count past 6 ticks; if you need 25 mm, that is 1 inch flat. Memorize: 1/4 in = 6.35 mm, 1/2 in = 12.7 mm, 1 in = 25.4 mm.
Millimeters in practice
Engineering drawings worldwide standardize on millimeters, regardless of the home country. The ISO drawing convention assumes mm unless otherwise specified. US drawings sometimes mix mm and inches in a single sheet, which is the kind of compromise that the Mars Climate Orbiter mishap was supposed to prevent. NASA now mandates single-system drawings on space hardware.
Optics and photography use millimeters universally. A 50 mm lens, a 200 mm telephoto, a 24-70 mm zoom: every camera maker on the planet uses the same scale. The number refers to focal length, not physical lens diameter, but it is still in millimeters. Microscopy crosses into micrometers (1 mm = 1,000 microns) and below, where the optical limits of visible light start to bite at around 0.5 micron.