Millimeter is 0.001 meter units.
Millimeters to Inches Converter — mm to in
Convert Millimeter (mm) to Inch (in) using the exact conversion factor (1 millimeter = 0.0393700787 inch). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Millimeters to Inches converter
Converter
Length Converter
Convert SI, imperial, US customary, astronomical, scientific, and historical length units.
Context values
Some unit families need one extra value for a physically accurate result.
An inch is exactly 0.0254 meters.
Result = input x 0.001 / 0.0254.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Millimeters to Inches
Millimeter and Inch both measure length. Millimeter is typically associated with standard small/precision length unit worldwide, including in US engineering and manufacturing, while Inch is typically associated with united States and United Kingdom (small measurements, screen sizes, hardware).
Formula
inches = millimeters × 0.0393700787402
This factor comes from each unit's defined relationship to the category's base unit: 1 millimeter equals 0.001 base units, and 1 inch equals 0.0254 base units, so dividing one by the other gives the direct millimeter-to-inch factor of 0.0393700787402.
Simple example
1 mm × 0.03937007874 = 0.0393700787 in
1 millimeter = 0.0393700787 inches.
Real-world example
10 mm × 0.03937007874 = 0.3937007874 in
10 millimeters = 0.3937007874 inches.
Conversion table
| Millimeter (mm) | Inch (in) |
|---|---|
| 1 mm | 0.0393700787 in |
| 5 mm | 0.1968503937 in |
| 10 mm | 0.3937007874 in |
| 21.0975 mm | 0.8306102362 in |
| 42.195 mm | 1.661220472 in |
| 100 mm | 3.937007874 in |
Reverse conversion: Inch to Millimeter
0.0393700787 in × 25.4 = 0.999999999 mm
0.0393700787 inches = 0.999999999 millimeters.
millimeters = inches × 25.4
For a page dedicated to this direction, see Inch to Millimeter.
Millimeter vs. Inch
| Aspect | Millimeter | Inch |
|---|---|---|
| Family | SI derived (metric prefix) unit | US/imperial customary unit |
| Typical use | Engineering drawings and manufacturing tolerances worldwide (including in the US) | Screen and display size measurement worldwide (even in metric countries) |
| Where it's used | Standard small/precision length unit worldwide, including in US engineering and manufacturing | United States and United Kingdom (small measurements, screen sizes, hardware) |
Understanding the Millimeter
Millimeter (mm) measures distance. A millimeter is one-thousandth of a meter. It is classified as a SI derived (metric prefix) unit.
Where it's used: Standard small/precision length unit worldwide, including in US engineering and manufacturing
Uses of the Millimeter today
- Engineering drawings and manufacturing tolerances worldwide (including in the US)
- Rainfall measurement in most countries
- Small-scale everyday measurement (screen sizes, hardware)
History of the Millimeter
The millimeter follows from the meter via the 'milli-' SI prefix (10⁻³). It is notable as one of the few metric units in routine US technical/engineering use, since precision manufacturing and electronics are often specified in millimeters even in the US.
Historical origin. Millimeter was not the invention of a single named person; it developed through the process described above.
Understanding the Inch
Inch (in) measures distance. The international inch is defined as exactly 25.4 millimeters. It is classified as a US/imperial customary unit.
Where it's used: United States and United Kingdom (small measurements, screen sizes, hardware)
Uses of the Inch today
- Screen and display size measurement worldwide (even in metric countries)
- US/UK small-object and body measurement
- Hardware, tools, and pipe fittings (US)
History of the Inch
The inch traces to medieval England, traditionally defined as the length of three barleycorns, later standardized as 1/12 of a foot. The international inch (exactly 25.4 mm) was fixed by the same 1959 agreement that standardized the foot and yard.
Historical origin. Inch was not the invention of a single named person; it developed through the process described above.
Millimeter — sources
- BIPM SI Brochure — SI prefixes — Defines 'milli-' as the prefix for 10⁻³.
Inch — sources
- NIST — International yard and pound (1959) — Fixes the modern inch at exactly 25.4 millimeters.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many inches are in 1 millimeter?
1 millimeter equals 0.0393700787 inches, using the exact defined conversion factor rather than an estimate.
How do I convert millimeter to inch?
Multiply the millimeter value by 0.03937007874. The converter above does this instantly to whatever precision you set.
How do I convert inch back to millimeter?
Use the reverse factor: 1 inch equals 25.4 millimeters. You can also use the swap control in the converter above to flip the direction instantly.
What is a millimeter?
Millimeter (mm) measures distance. A millimeter is one-thousandth of a meter. It is classified as a SI derived (metric prefix) unit.
What is a inch?
Inch (in) measures distance. The international inch is defined as exactly 25.4 millimeters. It is classified as a US/imperial customary unit.
Is the millimeter to inch conversion exact?
Yes. The international inch is defined as exactly 25.4 millimeters. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
Where is the millimeter still used today?
Standard small/precision length unit worldwide, including in US engineering and manufacturing