Converters guide
Inches to millimeters: why 25.4 is not a rounding
A lot of “about 25 mm” advice is how a ¼-inch bolt becomes the wrong socket. Since 1959 the inch is defined as 25.4 millimeters exactly. The converter is just that multiply. The work is turning a fraction into a decimal first.
Updated 2026-09-19
The factor you can trust
millimeters = inches × 25.4. One inch is 25.4 mm, not 25, not 2.54 cm with a shrug. A 2-inch pipe stub is 50.8 mm. A 12-inch ruler is 304.8 mm.
If a drawing says 5/16 in, that is 0.3125 in × 25.4 = 7.9375 mm. Do the fraction before you reach for a rounded “8 mm” unless the spec allows it.
When “inch” is a nickname
Nominal pipe size, lumber “2×4,” and some screen diagonals are not the measured inch. Convert the number you measured with a caliper, not the catalog name.
US survey foot is a hair different from the international foot. For a bolt or a phone spec it never matters. For a long property line, use the survey page’s dictionary.
Key takeaways
- Multiply inches by 25.4. That factor is exact.
- Turn fractions into decimals before you convert.
- Do not convert a nickname (NPS, 2×4) as if it were a measurement.