How to convert inches to millimeters
To convert inches to millimeters, multiply each value by the exact factor in the box: 1 inch = 25.4 millimeters exactly. 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.
Millimeters are how most of the world's manufactured things are actually specified — camera lenses, phone dimensions, fasteners, sheet materials — while American drawings and product listings still arrive in inches, very often as fractions. Multiplying inches by the exact 25.4 turns one system into the other with no approximation anywhere in the chain, because 25.4 mm is not a measured estimate of an inch: since 1959 it is what an inch is defined to be.
The fractional sizes are the daily work: ½ in is 12.7 mm (the table starts there for exactly this reason), ¼ in is 6.35 mm, and a ⅜ in drill is 9.525 mm — close to, but not interchangeable with, a 10 mm drill. Product dimensions cross constantly: a 24 in monitor is 609.6 mm diagonally, a 12 in tile is 304.8 mm, and a phone listed as 6.1 in tall in its US spec is 154.94 mm in its global one. Enter fractions as decimals (0.5, 0.25, 0.375) and the conversion is done.
The common mistake is feeding the fraction in as written — typing “1/2” or reading 1½ in as 1.12 in — which produces confident nonsense. Convert the fraction to a decimal first: sixteenths are 0.0625 each, so 5/16 in is 0.3125 in, or 7.9375 mm. The second trap is nominal pipe and lumber sizes, where a “2-inch” pipe or a “2×4” board names a category, not a measurement (a dressed 2×4 is 1.5 × 3.5 in). This page converts the number you give it; check that the number is a real measurement before you rely on the millimeters.
Worked example
1 inch is 25.4 millimeters exactly: 1 × 25.4 = 25.4 — no rounding, no approximation, because the millimeter figure is the inch's own international definition.
How to use this converter
- Enter your value in inches in the box above — the result updates as you type.
- Read the converted value in millimeters 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 Inches to Millimeters conversion table below for other common values without typing each one.