Article — Inch Converter
Inch converter: every length unit anchored to the exact 25.4 mm/inch factor
An inch converter swaps inches for mm, cm, m, ft, or yd using one exact factor: 1 in = 25.4 mm. The 1959 International Yard and Pound Agreement fixed that value as a legal definition, so the conversion has zero measurement uncertainty. 12 inches equals 304.8 mm. 36 inches equals 914.4 mm or 0.9144 m. 100 mm equals 3.937 inches, which fractional carpenters call 3 15/16 in. Pick the target unit from the dropdown and the converter runs in both directions at any precision from 0 to 6 decimal places.
The default target is millimeters because that is the unit used in engineering drawings, CNC machining, hardware specs, and most international product data sheets. Switch to cm for body and apparel work, m for surveying and architecture, ft and yd for landscaping and US road signs.
The inch converter formula
Inches times 25.4 = mm. Inches times 2.54 = cm. Inches times 0.0254 = m. Inches divided by 12 = ft. Inches divided by 36 = yd. The mm form is exact to all decimals. Going the other way, divide the target value by the same factor to recover inches. The mm form is the canonical anchor; every other factor is mm divided by a power of 10 or multiplied by an integer.
1 in = 25.4 mm exact1 in = 2.54 cm exact1 in = 0.0254 m exact12 in = 1 ft = 304.8 mm exact36 in = 1 yd = 914.4 mm exact1 m = 39.3701 in derivedThe shortcut 25 mm per inch is 1.6 percent low. For body height that costs 3 cm at 180 cm. The shortcut 1 in equals roughly 2.5 cm works for sizing only. For any record-keeping, hardware, or trade use, type the exact 25.4 mm/inch factor.
Inch converter for millimeters
Most industrial drawings use mm, even in the US. ASME B89 dimensional inspection standards specify mm for tolerances tighter than 0.1 mm. CNC machine controls accept either inches or mm but most modern shops set them to mm because the resolution is finer per digit. A 1/4 in drill bit is 6.35 mm; a 1/2 in is 12.7 mm; a 3/4 in is 19.05 mm. Hardware bins everywhere stock both labelings.
The 25.4 mm/inch factor was chosen to be a round metric value, not the other way around. Pre-1959, the US inch was 25.4000508 mm and the UK inch was 25.3999778 mm. The 1959 treaty harmonised six English-speaking countries on the round number 25.4 mm, simplifying machine-tool catalogs by half a percent.
Fractional inch converter
Fractional inches dominate US carpentry, plumbing, and hardware. The standard fractions divide an inch into halves, quarters, eighths, and sixteenths. Each 1/16 in equals 1.5875 mm. 1/8 in is 3.175 mm; 1/4 in is 6.35 mm; 1/2 in is 12.7 mm; 3/4 in is 19.05 mm. For tighter work, machine shops go to 1/32 in (0.79 mm) or 1/64 in (0.40 mm). To convert a decimal back to a fraction, multiply by 16, round to the nearest integer, write as sixteenths. 0.74 in times 16 = 11.84, rounds to 12, so 12/16 = 3/4 in.
If a 1/16 in step is too coarse for your work, use 1/32 in. That gets you 0.79 mm steps, which matches the smallest mark on most metric tapes. For finer than 1/32, switch to mm and skip fractions entirely.
Inch converter in engineering drawings
ANSI Y14.5 and ASME Y14.5 are the US engineering-drawing standards. They allow either inches or mm but require the unit to be stated explicitly. Mixing is forbidden on the same drawing. European drawings under ISO 128 always use mm. When a US drawing has to be remanufactured in a European shop, the inch-to-mm conversion is run on every dimension. The 25.4 factor is exact, so tolerance bands convert cleanly.
The most common confusion is sheet thickness. US sheet metal is gauged (gauges 24, 22, 20, 18, 16), but the underlying thickness is in inches. 18 gauge steel is 0.048 in = 1.219 mm. European sheet metal is sold by mm directly: 1.2 mm, 1.5 mm, 2 mm, 3 mm. Converting between the two systems goes through the inch.
Inches vs imperial vs US customary
The inch is one unit in two parallel systems. US customary is the legal civilian system in the United States. Imperial is the legal civilian system in the United Kingdom and a few Commonwealth countries. Both systems define the inch as exactly 25.4 mm under the 1959 treaty, so the conversion is identical. Differences appear in larger units: the US gallon (3.785 L) is smaller than the imperial gallon (4.546 L), and the US ton (907 kg) differs from the imperial ton (1016 kg).
History of the inch
The word inch comes from the Latin uncia, meaning one-twelfth, the same root as ounce. The Romans split their foot into 12 unciae, the convention that carried into English. Medieval England defined the inch by three barleycorns laid end to end (1324, Edward II), giving a unit of about 25 mm. Scotland used the width of an adult thumb. By the late 1800s the US and UK both had their own legal inches that differed by a few millionths of a millimetre, enough to cause friction in transatlantic machine-tool catalogs. The 1959 treaty fixed the number at exactly 25.4 mm and ended the disagreement.
Rounding rules for inch conversion
For tape-measure work, 1 decimal place of inches (0.1 in = 2.54 mm) is enough. For machine-shop work, 3 decimals (0.001 in = 0.0254 mm) is standard, called a "thou" or "mil." Precision machining goes to 0.0001 in. Optical fabrication uses 0.00001 in (10 millionths of an inch). The 25.4 mm/inch factor is exact, so any rounding error is from your input value, not from the conversion.
Common inch converter mistakes
The first mistake is using 25 mm or 2.5 cm as a shortcut. The 1.6 percent error is acceptable for talking about clothing sizes but ruinous for hardware. A 1/2 in pipe at 12.5 mm is undersize by 0.2 mm, which leaks. Always use 25.4 mm/inch for any record-keeping or trade use.
The second mistake is treating decimal feet as feet and inches. 5.9 ft is not 5 ft 9 in. It is 5 ft 10.8 in, because 0.9 ft times 12 = 10.8 inches. Convert decimal feet by multiplying the fractional part by 12, not by reading the decimal digit as inches.