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Metric precision, imperial result — exact every time
Millimeters are the language of engineering drawings, 3D printing and precision manufacturing. When you need the inch equivalent — for US tools, fasteners or client deliverables — divide by 25.4. Enter mm and get the exact decimal inch value instantly.
The formula
in = mm ÷ 25.4
- mm — the length in millimeters.
- in — the equivalent length in inches.
- 25.4 — the exact mm per inch (1959 International Yard and Pound Agreement).
Alternatively: in = mm × 0.03937 (the reciprocal of 25.4).
Worked example
Using the default of 50 mm:
- Divide by 25.4: 50 ÷ 25.4 = 1.96850…
- Rounded to 3 decimal places: 1.969″.
So 50 mm ≈ 1.969 inches (just under 2″).
Conversion table
| Millimeters (mm) | Inches (in) | Nearest fraction |
|---|---|---|
| 1 | 0.039 | ≈ 1/25″ |
| 3.175 | 0.125 | 1/8″ |
| 6.35 | 0.250 | 1/4″ |
| 10 | 0.394 | ≈ 3/8″ |
| 12.7 | 0.500 | 1/2″ |
| 19.05 | 0.750 | 3/4″ |
| 25.4 | 1.000 | 1″ |
| 50 | 1.969 | ≈ 1 31/32″ |
| 100 | 3.937 | ≈ 3 15/16″ |
| 150 | 5.906 | ≈ 5 29/32″ |
| 200 | 7.874 | ≈ 7 7/8″ |
| 304.8 | 12.000 | 12″ (1 ft) |
History & standards
The millimeter is 1/1000 of the SI meter, which has been defined since 1983 by the speed of light. The inch originated in Anglo-Saxon and Roman units. The exact link between the two was established by the 1959 International Yard and Pound Agreement: 1 inch = 25.4 mm, making the conversion lossless. Prior to 1959 the US used a survey inch (25.4000508 mm) for cadastral work; NIST retired that in 2023. The international inch (25.4 mm exactly) now applies in all US contexts.
Common applications
- Engineering drawings. ISO and DIN drawings use mm; ANSI/ASME drawings use inches — conversion is constant.
- 3D printing. STL files dimensioned in mm need inch values for US users or imperial-configured printers.
- Woodworking. European hardware is in mm; US power tools and fittings list inch sizes.
- Photography & optics. Sensor sizes, filter threads and focal lengths appear in mm; US charts sometimes use inches.
- Automotive. Tire widths (e.g. 205/55 R16) are in mm; rim widths are listed in inches.
Reverse conversion
To convert inches to mm, multiply by 25.4: mm = in × 25.4. Use the inches to mm calculator.
Limitations & gotchas
- Fractional inches. The calculator returns decimal inches. To find the nearest fraction, compare the decimal to common fractions: 0.25 = 1/4, 0.5 = 1/2, 0.75 = 3/4.
- Nominal vs. actual sizes. A “10 mm wrench” fits a 10 mm bolt head — but US “3/8-inch” socket sets are nominally 9.525 mm, not exactly 10 mm.
- US survey inch (deprecated 2023). The international inch (25.4 mm) is now the sole US legal 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
Divide by 25.4: in = mm ÷ 25.4. For example, 50 mm ÷ 25.4 ≈ 1.969 inches. The factor 25.4 is exact, fixed by the 1959 International Yard and Pound Agreement.
Exactly 25.4 mm. This is the exact internationally agreed definition, not an approximation.
25.4 ÷ 25.4 = 1 inch exactly.
50 ÷ 25.4 ≈ 1.969 inches (just under 2″).
10 ÷ 25.4 ≈ 0.394 inches (just under 3/8″).
The ratio 1″ = 25.4 mm is exact. The division 1÷25.4 = 0.039370… is an infinite decimal, so displayed results are rounded. Multiply your rounded answer by 25.4 to verify — you’ll get back the original mm value to the displayed precision.