Convert millimeters to inches
1 mm = 0.039370079 in · Formula and table below. Converting the other way? Inches to Millimeters →
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Other units
- Meter (m) 0.001
- Kilometer (km) 0.000001
- Decimeter (dm) 0.01
- Centimeter (cm) 0.1
- Millimeter (mm) 1
- Micrometer (µm) 1000
- Nanometer (nm) 1,000,000
- Mile (mi) 6.213712×10⁻⁷
- Yard (yd) 0.001093613
- Foot (ft) 0.00328084
- Inch (in) 0.039370079
- Nautical mile (nmi) 5.399568×10⁻⁷
- Light year (ly) 1.057001×10⁻¹⁹
Reviewed on by the Skala Convert team
Divide millimetres by 25.4. Ten millimetres is 0.394 inches.
How many inches is one millimeter?
in = mm ÷ 25.4
The answer is almost always a fraction in disguise. A workshop does not want 0.394 inches — it wants to know that 10 mm sits between 3/8 (9.525 mm) and 13/32 (10.32 mm). Converting metric to imperial in a practical setting means finding the nearest usable fraction.
Anchor: 25 mm is 0.984 inches, just under an inch — so a metre stick and a yardstick diverge slowly but visibly.
Why is a 1-inch camera sensor not 25.4 mm?
Lens focal lengths are metric everywhere — 50 mm, 85 mm, 200 mm — while sensor sizes are quoted in fractional inches (1/2.3", 1", APS-C). The inch measurements are not even real inches; they are a legacy of vacuum tube diameters from the 1950s, and a "1 inch" sensor is about 16 mm across. It is the one place where converting the number gives you an answer that is technically correct and practically meaningless.
Why is the divisor exactly 25.4?
Because the inch has been defined from the metre since 1959, when the US National Bureau of Standards (now NIST), in an announcement effective 1 July 1959 set it at 25.4 mm. NIST Special Publication 811, the US guide to the SI lists it as exact. On the metric side, the millimetre is a thousandth of a metre under the SI prefixes defined in the SI Brochure published by the International Bureau of Weights and Measures (BIPM).
Formula
Inches = Millimeters × 0.039370079
10 mm = 0.393700787 in
Conversion table
| mm | in |
|---|---|
| 1 | 0.039370079 |
| 2 | 0.078740157 |
| 5 | 0.196850394 |
| 10 | 0.393700787 |
| 20 | 0.787401575 |
| 25 | 0.984251969 |
| 50 | 1.968504 |
| 100 | 3.937008 |
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Frequently asked questions
What is 10 mm in inches?
0.3937 in, or roughly 25/64. The nearest common size is 3/8 in (9.525 mm).
Why is a "1 inch" camera sensor not 25.4 mm?
The designation comes from old video tube diameters, not from the sensor itself. A 1-inch sensor measures about 16 mm diagonally.
How many mm in a quarter inch?
6.35 mm.