Convert Inches to Millimeters
Multiply any inch value by 25.4 to get millimeters — since one inch is defined as exactly 25.4 mm, the conversion carries no rounding error.
- Millimeter (mm)25.4
- Centimeter (cm)2.54
- Meter (m)0.0254
- Kilometer (km)0.000025
- Inch (in)1
- Foot (ft)0.083333
- Yard (yd)0.027778
- Mile (mi)0.000016
- Ja/Chuk (자)0.083828
- Chi/Chon (치)0.838284
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The formula and why it's exact
mm = inches × 25.4. Since the 1959 International Yard and Pound Agreement, one inch is defined as exactly 25.4 mm, so this conversion involves no approximation. For example, 6 in = 6 × 25.4 = 152.4 mm, and 0.5 in = 12.7 mm. To go the other way, divide millimeters by 25.4 (100 mm ÷ 25.4 ≈ 3.937 in). Unlike temperature, length is a pure proportional relationship anchored at zero — there is no offset to add, so scaling by a single factor is all you ever need.
Fractional inches and real examples
US drawings often list inches as fractions like 1/2, 1/4, 1/8, or 1/16. Convert the fraction to a decimal first, then multiply by 25.4: 3/4 in = 0.75 × 25.4 = 19.05 mm, 1/8 in = 3.175 mm, and 3/16 in = 4.7625 mm. A 5.5-inch phone screen is 139.7 mm, a 27-inch monitor diagonal is 685.8 mm, and a 4×6 photo prints at 101.6 × 152.4 mm. You'll reach for this constantly when moving imperial woodworking, 3D-printing, or CAD plans onto metric equipment.
Watch out: rounding and nominal sizes
Results rarely land on round numbers — 1/16 in works out to 1.5875 mm, so round to 0.1 mm or 0.01 mm depending on your machining tolerance. Labels like "2×4 lumber" or "3/4-inch pipe" are nominal sizes, not actual measurements, so don't multiply them by 25.4 directly (a real 2×4 cross-section is about 38 × 89 mm). Finally, the deprecated US survey inch equals 25.4000508 mm — very slightly larger than the international inch. That gap only matters over kilometers in surveying, and it was officially retired at the end of 2022.
Frequently asked questions
How many millimeters is exactly one inch?
Exactly 25.4 mm. This isn't a rounded figure — it's the definition fixed by the 1959 international agreement, so inches × 25.4 always gives an exact result. For instance, 10 in = 254 mm and 2.5 in = 63.5 mm.
How do I convert millimeters back to inches?
Divide by 25.4: mm ÷ 25.4 = inches. So 50 mm ÷ 25.4 ≈ 1.9685 in and 300 mm ≈ 11.811 in. Inches are usually written to three decimal places or rounded to the nearest common fraction, such as 1/16.
Why do some conversions produce long decimals?
Because 25.4 is a terminating decimal, whole-inch inputs stay tidy, but fractional inches don't. A value like 1/16 in becomes 1.5875 mm, adding several digits. In practice, rounding to one or two decimal places to match your drawing's tolerance is more than enough.