Inches to millimeters converter
Convertersconversion pair
Convert inches to millimeters instantly: 1 inch is exactly 25.4 mm. Bolts, screens and drill bits, with the exact math and one-click copying.
an inch is 25.4 mm — the only imperial number the metric system kept.
work surface
Math, not advice — results are indicative. Read the disclaimer.
pro tips
Rough math: multiply by 25. A 4-inch pipe is about 100 mm.
1/8 inch is 3.175 mm — the decimal hiding inside every fraction.
A "2x4" is really 1.5 by 3.5 inches, which is 38 by 89 mm. Lumber lies in both systems.
what it does
One inch is exactly 25.4 millimeters. No rounding, no argument — that exact figure was agreed in 1959 and nobody has complained since. Multiply inches by 25.4 and the job is done.
Where it matters: drill bits (a quarter-inch bit is 6.35 mm, and the hole notices), screens quoted in inches but cut to millimetre tolerances, bicycle and car parts that blend standards on purpose, and any hardware drawer that speaks in fractions while the label whispers in millimetres.
The shortcut rounds ugly on purpose: 1 inch is about 25 mm, 4 inches about 100 mm, 10 inches about 254 mm. Fine for a glance, useless for a fit. If the part has to slot into something, trust the exact number, not the vibe.
Type any inch value — decimals welcome — and the widget answers in millimetres instantly. Click to copy, and use swap when a spec sheet hands you millimetres and your ruler is still in inches.
quick answers
How many millimeters is half an inch?
12.7 mm exactly. Halve 25.4 and you've halved the confusion too.
Is 25.4 exact or rounded?
Exact. It has been the legal definition of the inch since 1959.
Why do screens use inches if the world is metric?
Marketing found "diagonal in inches" easier to sell, and it stuck. The panel itself is still cut in millimetres.