Article — mm to Inches Conversion
mm to inches: convert millimeters to inches exactly
One inch equals exactly 25.4 millimeters. One millimeter equals 0.03937 inches. The first number is the legal definition of an inch, fixed by international treaty in 1959. Every length conversion between metric and imperial on this page derives from those two values, and they are exact — no rounding involved.
The calculator at the top of this page does the math in either direction. The article below explains where 25.4 comes from and the most common places mm-to-inch conversions show up.
What is a millimeter?
A millimeter is one-thousandth of a metre. The metre is the SI base unit of length, defined since 1983 by the distance light travels in vacuum in 1/299,792,458 of a second. The millimeter is the everyday metric unit for small measurements — paper thickness, hardware tolerances, lens specifications, jewelry sizing.
Most of the world uses millimeters and centimeters for small measurements. The United States, United Kingdom, and a handful of other countries continue to use inches alongside the metric system, especially in construction, hardware, and consumer products.
What is an inch?
An inch is a unit of length equal to exactly 25.4 millimeters. The international agreement of 1959 made this exact, replacing slightly different US and imperial inches that existed before. The new "international inch" applies in the US, UK, Canada, Australia, New Zealand, and South Africa for most modern purposes.
The inch survives in the US for retail products (TVs measured diagonally, monitors, phone screens), construction (lumber, drywall), and hardware (nuts, bolts, drill bits). In the UK it survives in road signs (miles), height measurements ("she is 5 foot 7"), and a few legacy industries.
The mm to inches formula
Two-way conversion is one division or one multiplication:
inches = mm ÷ 25.4 (exact)mm = inches × 25.4 (exact)mm ÷ 25 (mental math, 1.6% off)inches × 25 (reverse mental math)The factor 25.4 is exact and never needs more than one decimal place — that is the whole number that the 1959 treaty wrote into the definition.
Common mm conversions
The most-searched values, at the exact 25.4 factor:
- 1 mm = 0.0394 in (a thick paper)
- 10 mm = 0.3937 in (1 cm, a US dime's diameter is close to this)
- 25.4 mm = exactly 1 in
- 50 mm = 1.9685 in (about 2 inches)
- 100 mm = 3.9370 in (about 4 inches)
- 250 mm = 9.8425 in (about 10 inches)
- 304.8 mm = exactly 12 in = 1 foot
- 914.4 mm = exactly 36 in = 1 yard
- 1,000 mm = 39.3701 in (1 metre)
Fractional inches and hardware sizes
US hardware uses fractional inches: 1/16, 1/8, 1/4, 1/2, 3/4. Each fractional inch has an exact mm equivalent:
1/4-inch is 6.35 mm. 1/2-inch is 12.70 mm. 3/4-inch is 19.05 mm. The series doubles cleanly because the inch divides into halves recursively. Metric hardware uses round mm values (M3, M4, M6, M8, M10, M12) that do not align with fractional inch sizes — which is why an imperial drill index and a metric one cover different sets of holes.
For drilling pilot holes, the 25 mm ≈ 1 in approximation works fine. For tapping threads, use the exact 25.4 factor and the tap manufacturer's chart. A 0.05 mm error on a tap drill can mean the difference between a tight thread and a loose one.
Why exactly 25.4 mm?
Before 1959 the US and imperial inches differed by a tiny amount. The US inch was defined indirectly through the 1893 Mendenhall Order, which made the US yard equal to 3600/3937 metres. That works out to one inch ≈ 25.4000508 mm. The British imperial inch was defined separately from a physical reference bar and came out at about 25.39977 mm.
The 0.0003 mm gap was invisible in everyday work but caused real problems in international engineering. So the 1959 International Yard and Pound Agreement defined a new "international inch" equal to exactly 25.4 mm, between the old US and imperial values. Both countries adopted it for industrial purposes.
The US still recognises the older "US survey inch" (25.4000508 mm) for federal land surveys, where consistency with historical 19th-century records is more important than alignment with the international standard. NIST formally deprecated the US survey foot in December 2022, ending a century-long parallel system.
Common mm-to-inch mistakes
Confusing mm with cm. 100 mm = 10 cm, not 100 cm. A "100 mm" measurement is roughly 4 inches; "100 cm" is just over 3 feet.
Treating fractional inches as decimal. 1/4 inch is 0.25 inch, not 1.25 inch. The fractional notation throws people who only see decimals.
Using metric drill bits on imperial holes (and vice versa). M6 (6 mm) is close to but not identical to 1/4 inch (6.35 mm). A 6 mm bolt does not fit cleanly in a 1/4-inch hole. Match the system end-to-end.
Rounding 25.4 to 25. Fine for casual estimates, wrong for precision machining. The 1.6% error accumulates fast on longer measurements.