Mm to Inches: Precise Conversion for Engineering and Craft
Convert millimeters to inches with the exact 1 in = 25.4 mm ratio, plus a reference table and where mm-precision actually matters.
For anything that needs precision — machine screws, drill bits, phone screens, camera lenses, jewellery — millimeters and inches are the working units. The conversion is one of the cleanest in the whole unit system, because since 1959 the inch has been *defined* as exactly 25.4 mm.
The formula
inches = millimeters ÷ 25.4 millimeters = inches × 25.4
1 in = 25.4 mm exactly. That value is legally fixed by the international yard and pound agreement; there is no rounding, and no country in the modern world uses a different value.
*Example:* a common wood screw is 3.5 mm across. 3.5 ÷ 25.4 = 0.1378 in — roughly a #6 screw in the US size system.
Reference table
| Millimeters | Inches | Everyday reference |
|---|---|---|
| 1 mm | 0.0394 in | Pencil lead |
| 3 mm | 0.1181 in | Lego brick wall thickness |
| 5 mm | 0.1969 in | Small drill bit |
| 10 mm | 0.3937 in | 1 cm |
| 25.4 mm | 1 in | One inch, exactly |
| 50 mm | 1.9685 in | 2-inch drainpipe |
| 100 mm | 3.9370 in | Small tile |
| 300 mm | 11.811 in | Standard ruler |
For any custom precision value, the Mm to Inches converter handles it without cumulative rounding.
Fractional inches — where it gets awkward
Imperial workshops still often work in fractions of an inch (1/8", 1/16", 1/32"). The mm-to-inch equivalents rarely land on clean fractions:
| Millimeters | Nearest fraction | Decimal inches |
|---|---|---|
| 3.175 mm | 1/8 in | 0.1250 in |
| 6.35 mm | 1/4 in | 0.2500 in |
| 12.7 mm | 1/2 in | 0.5000 in |
| 19.05 mm | 3/4 in | 0.7500 in |
| 25.4 mm | 1 in | 1.0000 in |
So a "half-inch bolt" is 12.7 mm, not 12 or 13. Metric plumbing and imperial plumbing don't interchange cleanly for this reason, and drill bit sets sold internationally usually come in both fractional and metric versions.
Where 25.4 becomes visible in real life
- Screens. A "24-inch monitor" measured diagonally is 609.6 mm. Panel manufacturers list both.
- Camera sensors and lenses. A "35 mm" focal length is genuine millimeters. Sensor size is often given in inches — but those are a marketing convention from old TV tube sizes and don't match physical inches at all.
- Machining. Modern CNC machines work in either unit natively; drawings must state "mm" or "in" in the title block. The ISO GPS standard covers metric drawing conventions.
- Firearms. Cartridge calibres mix units freely: a ".22 LR" is a nominal 0.22-inch bore (5.6 mm); "9 mm" is genuinely 9 mm.
Precision matters more than the ratio
The bigger source of error in mm/inch work isn't the conversion itself — it's *when* you round. If you're chaining conversions (mm → in → back to mm), always carry full precision internally and round only for the final display. That's how good workshops avoid the "1 mm gap that shouldn't be there".
Reference-grade conversion practices are documented in NIST Special Publication 811, the standard US guide for SI units.
FAQ
How many millimeters is 1 inch? Exactly 25.4 mm. No approximation.
Is 10 mm exactly 1 cm? Yes — the centimeter is defined as 10 millimeters. So 10 mm = 1 cm = 0.3937 in.
Why does mm-to-inch matter more than cm-to-inch in engineering? Because CAD, machining, and precision manufacturing express tolerances in fractions of a millimeter. Working in centimeters would need too many decimal places and invites rounding errors.
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*For workshop-grade conversions with no rounding shortcuts, use the free Mm to Inches Converter.*