Calculator
What the Inch to MM does
Inch to MM: 1 in = 25.4 mm, so multiply a inch value by 25.4 to get millimetre, or divide to go back. Example: 1 in = 25.4 mm.
Give the Inch to MM your Value, From and To and it works out Result, How it was calculated and Reverse conversion. Everything recomputes live, which makes it easy to test a few scenarios in a row.
Converting once from the original figure beats chaining approximations. The Inch to MM exists to take that particular chore off your hands.
Your data stays on your machine. Once the page has loaded you could disconnect entirely and it would still work.
The field table, the method and a worked example are documented after the tool for anyone who wants to check the working.
What do the Inch to MM fields mean?
The Inch to MM uses 3 inputs. It opens with a complete example loaded, which is usually the quickest way to understand what a field means.
| Field | What to enter | Default |
|---|---|---|
| Value | required | 1 |
| From | choose from Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m) | Inch (in) |
| To | choose from Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m) | Millimetre (mm) |
How does the Inch to MM work?
Both units are converted through length's base unit (m), so any of the 19 units in the dropdowns can be paired in either direction without loss of accuracy.
Every factor here is the defined one, not a textbook approximation. That matters more than it sounds: rounding a factor early is the usual reason two converters disagree in the fourth decimal place.
Worked example
These are the values the Inch to MM loads by default, and the result it produces from them. The numbers below come straight from the default inputs, so you can verify them against the live tool.
| Value | 1 |
|---|---|
| From | Inch (in) |
| To | Millimetre (mm) |
| Result | 25.4 mm |
|---|---|
| How it was calculated | 1 in = 25.4 mm, so 1 × 25.4 = 25.4 mm |
| Reverse conversion | 0.039370079 in (if you meant 1 mm) |
Inches and millimetres
1 inch equals exactly 25.4 millimetres: an exact definition, so converting inches to mm is pure multiplication with no rounding error introduced by the constant itself.
Worth knowing
- Fastener and drill-bit sizing is where this conversion matters most in practice: US hardware is sized in fractional inches, while most of the world (and most 3D-printer/CNC tooling) works in mm, so a "1/4 inch" bolt is exactly 6.35 mm, not the 6 mm bolt that looks almost identical but is a different standard entirely.
- Screen and camera-sensor specifications sometimes mix the two: a "1-inch" camera sensor is not actually 25.4 mm across. It is a legacy naming convention from vacuum-tube camera sizes and refers to a sensor closer to 16 mm.
- Because 25.4 does not divide evenly into most round mm numbers, converting a "nice" inch fraction (like 1/8") to mm rarely produces a clean decimal: 1/8" is 3.175 mm exactly, which most drawings round to 3.2 mm.
Definitions: BIPM SI Brochure · NIST reference on units
How to use it
- Set Value.
- Pick a value for From: the options are Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m).
- Pick a value for To: the options are Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m).
- Skip the submit button; there is not one. Editing is enough.
- Check Result, How it was calculated and Reverse conversion on the right.
- To keep the result, either copy the link or print the page. Both preserve what is on screen.
What this tool does not do
- Conversion factors use SI definitions; some industries use their own rounded factors.
- Repeated conversions compound rounding, so convert once from the original value where possible.
- No allowance is made for local rules, regional variations or house conventions.
- The tool trusts your numbers completely, so a mis-keyed digit propagates straight through.
Frequently asked questions
Value, From and To, nothing else. Defaults are loaded for all of them, so nothing is mandatory before you see a result.
With the values loaded when the page opens, result comes out as 25.4 mm. Try changing one value. The answer refreshes without a page reload.
No. There is no account, no session and no storage. The values exist only while the tab is open.
It is free with no conditions: no sign-up, and no limit on how often you use it.
The formula is shown above precisely so you do not have to take the accuracy on trust.