How to convert millimeters to inches
To convert millimeters to inches, divide each value by the exact factor in the box: 1 millimeter = 0.0393700787 inches (exactly 1 in = 25.4 mm). There is no rounding anywhere in the chain — the display trims the final answer to six decimal places, and converting the result back with the inverse page returns your starting number.
The millimeter is the working unit of engineering drawings, manufacturing and most of the world's hardware: bolt diameters, sheet thicknesses, drill sizes and tolerances are all specified in millimeters, a thousandth of a meter. American workshops, however, still speak in inches — and in fractions of inches at that — so drawings, parts and fasteners constantly need translating between the two, at precisions where a sloppy factor stops being a rounding issue and becomes a part that does not fit.
Thread and drill sizes are the classic job. An M6 bolt has a 6 mm major diameter, 0.23622 in; a 10 mm drill is 0.39370 in, close to (but not) a ⅜ in drill at 0.375 in; and the 25 mm default here is 0.98425 in, noticeably not the “about an inch” it is often sold as — the missing 0.4 mm is exactly the kind of gap that makes a shaft bind. Film and photo sizes (35 mm = 1.378 in), rain gauges and coin diameters all convert the same way.
The common mistake is rounding mid-job: converting each dimension of a part with “divide by 25” and compounding the error across a drawing, when the exact divisor is 25.4. The second trap is the decimal-to-fraction jump — machinists read 0.98425 in as 63/64 in, not as “about 1 in”, and choosing the nearest fraction is a judgement this page deliberately leaves to you: it gives the exact decimal, you pick the drill. Keep full precision until the final step and the part fits first time.
Worked example
25 millimeters is 0.984252 inches: 25 ÷ 25.4 = 0.984252 — close to an inch, but 0.4 mm short of it, a difference that matters for the bolts, drills and bearings this conversion usually serves.
How to use this converter
- Enter your value in millimeters in the box above — the result updates as you type.
- Read the converted value in inches in the headline panel.
- Check the factor, the formula and the inverse result (your answer converted back) in the stats if you need to show your working.
- Use the Millimeters to Inches conversion table below for other common values without typing each one.