Inches to Millimetres Converter (in to mm)
Convert inches to millimetres instantly. 1 in = 25.4 mm. Includes the formula, a full conversion table and a calculator that works offline.
The formula
To convert inches to millimetres, multiply by 25.4:
mm = in × 25.4Going the other way, divide instead — or multiply by 0.0394, which is the same thing:
in = mm × 0.0394Inches to Millimetres conversion table
| Inches | Millimetres |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 30 in | 762 mm |
| 40 in | 1,016 mm |
| 50 in | 1,270 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
| 250 in | 6,350 mm |
| 500 in | 12,700 mm |
| 1000 in | 25,400 mm |
Millimetres to Inches
| Millimetres | Inches |
|---|---|
| 1 mm | 0.0394 in |
| 2 mm | 0.0787 in |
| 3 mm | 0.1181 in |
| 4 mm | 0.1575 in |
| 5 mm | 0.1969 in |
| 6 mm | 0.2362 in |
| 7 mm | 0.2756 in |
| 8 mm | 0.315 in |
| 9 mm | 0.3543 in |
| 10 mm | 0.3937 in |
What that actually looks like
A conversion factor is hard to judge on its own. These are quantities most people already have a feel for, converted for you:
- A standard A4 sheet is 8.27 inches wide — 210.06 mm
- A 15-inch laptop screen measures diagonally — 381 mm
- A US letter page is 11 inches tall — 279.4 mm
- A typical door is 32 inches wide — 812.8 mm
Why both units are still in use
Almost every country measures length in metres and centimetres. The United States is the significant exception, and the United Kingdom is genuinely split: road signs are in miles, a person's height is in feet and inches, but a bottle of water is in millilitres and a running race is in kilometres. That is why this conversion comes up so often — not because one system is replacing the other, but because both are in daily use side by side.
Mistakes worth avoiding
The most common mistake is mixing feet and inches in one number. Five foot ten is 5.833 feet, not 5.10 — the inches are twelfths, not hundredths. Convert the inches separately, or work in inches throughout and divide at the end.
The other habit worth breaking is rounding too early. Convert first, round once at the end, and you avoid the drift that turns a small approximation into a visible error by the third calculation.
Doing it in your head
When you only need a rough answer, multiply by 25 — which lands within 1.6 percent of the true value. Ten inches works out at roughly 250 millimetres that way, against 254 exactly. An estimate is enough for deciding whether something will fit or whether a figure sounds plausible. When the number has to be right — because you are buying, cooking or building to it — use the calculator and let it carry the decimals.
Common questions
How many millimetres are in one inch?
One inch equals 25.4 millimetres. The calculator above handles any value, including decimals and very large numbers, and updates as you type.
Is this an exact conversion?
Yes — this is an exact defined ratio, not an approximation. The table rounds for readability; the calculator does not.
Does this work offline?
Yes. The conversion runs in your browser with no network request, so it works on a plane, in a basement, or on a phone with no signal once the page has loaded.