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Precision conversion for DIY, machining and 3D printing
Inches to millimeters is the go-to conversion when you’re mixing imperial tools with metric drawings — or vice versa. The conversion is exact (1 in = 25.4 mm) so there is no rounding error. Enter inches and get the exact millimeter equivalent instantly.
The formula
mm = in × 25.4
- in — the length in inches.
- mm — the equivalent length in millimeters.
- 25.4 — the exact factor (1959 International Yard and Pound Agreement).
Reverse: in = mm ÷ 25.4.
Worked example
Using the default of 2 inches:
- Multiply by 25.4: 2 × 25.4 = 50.8 mm.
So 2″ = 50.8 mm exactly.
Common fractional inch values in mm
| Inches (fraction) | Inches (decimal) | MM |
|---|---|---|
| 1⁄16″ | 0.0625 | 1.5875 |
| 1⁄8″ | 0.125 | 3.175 |
| 3⁄16″ | 0.1875 | 4.7625 |
| 1⁄4″ | 0.25 | 6.35 |
| 3⁄8″ | 0.375 | 9.525 |
| 1⁄2″ | 0.5 | 12.7 |
| 5⁄8″ | 0.625 | 15.875 |
| 3⁄4″ | 0.75 | 19.05 |
| 7⁄8″ | 0.875 | 22.225 |
| 1″ | 1.0 | 25.4 |
Whole-inch conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127.0 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254.0 |
| 12 | 304.8 |
| 24 | 609.6 |
| 36 | 914.4 |
History & standards
The millimeter is 1/1000 of the meter, the SI base unit of length. The inch traces to the Roman “uncia” (1/12 of a foot). The exact conversion — 1 in = 25.4 mm — was fixed by the 1959 International Yard and Pound Agreement between the US, UK, Australia, Canada, New Zealand and South Africa. Prior to 1959 the US used a slightly different conversion (1 in = 25.4000508 mm for survey work). NIST retired this US survey inch in 2023; the international definition applies universally.
Common applications
- Woodworking & joinery. Router bits and drill bits are sized in inches in the US; metric plans list mm.
- 3D printing. Many printers use mm-based firmware (Marlin, Klipper); model dimensions from US sources are in inches.
- Machining. CNC G-code can work in either inches or mm; converting between the two is routine.
- Pipe & fastener sizing. US nominal pipe diameters are in inches; metric equivalents are in mm.
- Photography. Film formats (35 mm, 120) use mm; older US lenses and enlargers use inches.
Reverse conversion
To convert mm to inches, divide by 25.4: in = mm ÷ 25.4. Use the mm to inches calculator.
Limitations & gotchas
- Fractional inches. Enter as decimals (e.g. 0.5 for ½″). See the fractional-inch table above for common values.
- Nominal vs. actual sizes. A “2×4” lumber board is actually 1.5″ × 3.5″; a “1/2-inch pipe” has a bore of ~0.622″. Always measure rather than assume nominal sizes.
- US survey inch (deprecated 2023). The international inch is now the sole legal US definition.
Sources & references
- NIST SP 811, Guide for the Use of the International System of Units (SI).
- BIPM, The International System of Units (SI), 9th edition.
- 1959 International Yard and Pound Agreement.
- Federal Register Vol. 88, No. 60 (2023) — deprecation of the US survey foot/inch.
FAQs
Multiply by 25.4: mm = in × 25.4. For example, 2 in × 25.4 = 50.8 mm. The factor 25.4 is exact, defined by the 1959 International Yard and Pound Agreement.
Exactly 25.4 mm. This is a legally defined exact value, not an approximation.
1 × 25.4 = 25.4 mm exactly.
2 × 25.4 = 50.8 mm.
6 × 25.4 = 152.4 mm — which is also exactly 1 foot (0.3048 m × 1000).
Yes. The 1959 International Yard and Pound Agreement defined 1 inch = 25.4 mm as an exact value. There is no rounding in the underlying calculation — only in the displayed result.