Convert inches to mm

1. How to convert inches to mm

The conversion is a single multiplication: mm = in × 25.4. Both units are defined against the same base — the metre (m) — so the factor is fixed and the result is exact to the precision shown.

Worked at the two values people ask for most: 1 in = 25.4 mm and 5 in = 127 mm. Going the other way, in = mm × 0.0393700787402, so 1 mm = 0.039370079 in — a direction with its own page, mm to inches.

Results are given to eight significant figures. That is more precision than most jobs need — for a rough figure, use 1 in = 25.4 mm and adjust — but rounding early and multiplying afterwards is how a small error becomes a visible one, so the full figure is what the converter carries.

2. Inches to Millimetres conversion table

Every row is the same factor applied — nothing in the table is a special case, and any value in between behaves the same way. Type your own figure into the converter above and the Formula tile shows the arithmetic being used, so the result is checkable rather than taken on trust.

3. What the two units are

• Inches (in). One in is 0.0254 m.
• Millimetres (mm). One mm is 0.001 m.

Both are converted through the metre (m): the value is multiplied by its own unit's factor to reach the base, then divided by the target unit's factor to leave it. No direct inches to mm factor has to exist or be kept correct, which is also why the converter can show the same value in all 10 length & distance units at once.

The metre is the base and every length unit is an exact multiple of it by definition: an inch is exactly 25.4 mm, a foot exactly 0.3048 m, a yard exactly 0.9144 m, a mile exactly 1,609.344 m and a nautical mile exactly 1,852 m. None of those are measurements — they are agreements, which is why the conversion is exact rather than approximate.

If the pair you actually want is not this one, the full unit converter runs every 10 length & distance units off the same factor table, along with 11 other categories of measurement.