Millimeters to Inches Converter
Enter a length in millimeters and the converter returns inches straight away, rounded to four decimal places. One inch is exactly 25.4 mm, and the guide shows how to apply that by hand.
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Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
How many millimeters are in an inch?
One inch is 25.4 millimeters, and the number is exact. The international yard and pound agreement of 1959 fixed the inch at 25.4 mm for the English-speaking countries, so the relationship is a definition rather than a measured approximation.
1 in = 25.4 mm 1 mm = 1 ÷ 25.4 in ≈ 0.03937 inThe reciprocal never terminates, which is why millimeter-to-inch answers are rounded.
A handy anchor: 12.7 mm is half an inch, and 6.35 mm is a quarter inch.
Millimeters and inches compared
The millimeter belongs to the metric system and is the default small-length unit in manufacturing, electronics and most of the world. The inch belongs to the imperial and US customary systems. It is still the everyday unit in the United States for screen sizes, lumber, pipe and tool dimensions, and it is divided into binary fractions such as 1/8, 1/16 and 1/32 rather than decimals.
You meet both on the same job all the time: a metric bolt going into an imperial bracket, a camera lens spec in millimeters against a screen listed in inches, or a part drawing from overseas. Inch fractions convert to awkward decimals in millimeters, which is one reason a calculator helps. For instance 1/8 in is 3.175 mm and 3/8 in is 9.525 mm.
How to convert mm to inches
inches = millimeters ÷ 25.4
- Type the length in millimeters into the first box.
- The inches value shows in the second box and updates as you edit.
- By hand, divide by 25.4. For a rough estimate you can multiply by 0.04, remembering that the true factor (0.03937) is slightly smaller.
Worked examples
35 mm film. The film is 35 mm wide: 35 ÷ 25.4 = 1.3780 in, so about 1.378 in.
A ruler. A 150 mm ruler is 150 ÷ 25.4 = 5.9055 in long.
A4 paper. Its sides are 210 mm and 297 mm. That works out to 8.2677 in by 11.6929 in, close to but not the same as US Letter (8.5 × 11 in).
Reverse check. 3 inches × 25.4 = 76.2 mm, so converting 76.2 mm back returns exactly 3 in.
Millimeters to inches conversion table
| Millimeters (mm) | Inches (in) |
|---|---|
| 1 | 0.0394 |
| 2 | 0.0787 |
| 3 | 0.1181 |
| 5 | 0.1969 |
| 10 | 0.3937 |
| 12.7 | 0.5 |
| 20 | 0.7874 |
| 25.4 | 1 |
| 50 | 1.9685 |
| 75 | 2.9528 |
| 100 | 3.937 |
| 304.8 | 12 |
Rounding and where to go next
The converter keeps four decimal places, a step of 0.0001 in (about 0.0025 mm). Trailing zeros are dropped, so 25.4 mm displays as 1 rather than 1.0000. If you are cutting or machining to a tolerance finer than that, carry the full division instead of the rounded figure.
To go from inches back to millimeters use the inches to millimeters converter. For other metric steps see millimeters to centimeters and millimeters to meters; the full set sits under dimension converters.
Frequently asked questions
How many mm are in an inch?
Exactly 25.4 mm. That value has been fixed by international agreement since 1959.
How do I convert mm to inches?
Divide the millimeters by 25.4. For example, 50 mm ÷ 25.4 = 1.9685 in.
How many inches is 10 mm?
10 mm is 10 ÷ 25.4 = 0.3937 inches, a little under 2/5 of an inch.
Why does the converter show 0.0394 for 1 mm?
One millimeter is 0.03937 inches. The tool rounds to four decimals, which gives 0.0394.
References
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI)
- International Yard and Pound Agreement of 1959
Cite this converter and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Millimeters to Inches Converter. Reeliy. https://reeliy.com/converters/convert-mm-to-in-millimeters-to-inches“Millimeters to Inches Converter.” Reeliy, 2026, https://reeliy.com/converters/convert-mm-to-in-millimeters-to-inches.