1. How to convert mm to inches
The conversion is a single multiplication: in = mm × 0.0393700787402. 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 mm = 0.039370079 in and
5 mm = 0.19685039 in. Going the other way, mm = in × 25.4, so
1 in = 25.4 mm — a direction with its own page, inches to mm.
Results are given to eight significant figures. That is more precision than most jobs need — for a rough figure, treat 1 mm as about 0.039370079 in 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. Millimetres to Inches conversion table
| Millimetres (mm) | Inches (in) |
|---|---|
| 1 mm | 0.039370079 in |
| 2 mm | 0.078740157 in |
| 3 mm | 0.11811024 in |
| 5 mm | 0.19685039 in |
| 10 mm | 0.39370079 in |
| 12 mm | 0.47244094 in |
| 20 mm | 0.78740157 in |
| 25 mm | 0.98425197 in |
| 50 mm | 1.9685039 in |
| 100 mm | 3.9370079 in |
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
- Millimetres (mm). One mm is 0.001 m.
- Inches (in). One in is 0.0254 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 mm to inches 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.
4. Millimetres to Inches FAQs
How do I convert mm to inches?
Multiply by 0.0393700787402. 1 mm = 0.039370079 in, and 5 mm = 0.19685039 in. The factor is exact to the precision shown, so the only rounding is the one you choose to do at the end.
How many inches is 1 mm?
1 mm = 0.039370079 in. The full formula is in = mm × 0.0393700787402, and 1 in = 25.4 mm going the other way.
What is 5 mm in in?
5 mm = 0.19685039 in. The table above works the same factor through 10 values, and the converter takes any figure you type — including decimals.
How do I convert inches to mm?
mm = in × 25.4, so 1 in = 25.4 mm. Use the Swap button in the converter to reverse the direction without retyping the value, or open the dedicated inches to mm page.
Is the conversion exact or rounded?
Exact. Every imperial length unit is defined as an exact multiple of the metre — an inch is exactly 25.4 mm, a mile exactly 1,609.344 m — so the factor is a definition rather than a measurement. Results are shown to eight significant figures.
Is anything I type sent to a server?
No. The factor table ships with the page and every conversion runs in your browser. Nothing is uploaded, logged or written to storage, and the converter keeps working offline once the page has loaded.