An inch is exactly 0.0254 meters.
Inches to Millimeters Converter — in to mm
Convert Inch (in) to Millimeter (mm) using the exact conversion factor (1 inch = 25.4 millimeter). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Inches to Millimeters 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.
Millimeter is 0.001 meter units.
Result = input x 0.0254 / 0.001.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Inches to Millimeters
Inch and Millimeter both measure length. Inch is typically associated with united States and United Kingdom (small measurements, screen sizes, hardware), while Millimeter is typically associated with standard small/precision length unit worldwide, including in US engineering and manufacturing.
Formula
millimeters = inches × 25.4
This factor comes from each unit's defined relationship to the category's base unit: 1 inch equals 0.0254 base units, and 1 millimeter equals 0.001 base units, so dividing one by the other gives the direct inch-to-millimeter factor of 25.4.
Simple example
1 in × 25.4 = 25.4 mm
1 inch = 25.4 millimeters.
Real-world example
10 in × 25.4 = 254 mm
10 inches = 254 millimeters.
Conversion table
| Inch (in) | Millimeter (mm) |
|---|---|
| 1 in | 25.4 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 21.0975 in | 535.8765 mm |
| 42.195 in | 1,071.753 mm |
| 100 in | 2,540 mm |
Reverse conversion: Millimeter to Inch
25.4 mm × 0.03937007874 = 1 in
25.4 millimeters = 1 inch.
inches = millimeters × 0.0393700787402
For a page dedicated to this direction, see Millimeter to Inch.
Inch vs. Millimeter
| Aspect | Inch | Millimeter |
|---|---|---|
| Family | US/imperial customary unit | SI derived (metric prefix) unit |
| Typical use | Screen and display size measurement worldwide (even in metric countries) | Engineering drawings and manufacturing tolerances worldwide (including in the US) |
| Where it's used | United States and United Kingdom (small measurements, screen sizes, hardware) | Standard small/precision length unit worldwide, including in US engineering and manufacturing |
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.
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.
Inch — sources
- NIST — International yard and pound (1959) — Fixes the modern inch at exactly 25.4 millimeters.
Millimeter — sources
- BIPM SI Brochure — SI prefixes — Defines 'milli-' as the prefix for 10⁻³.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many millimeters are in 1 inch?
1 inch equals 25.4 millimeters, using the exact defined conversion factor rather than an estimate.
How do I convert inch to millimeter?
Multiply the inch value by 25.4. The converter above does this instantly to whatever precision you set.
How do I convert millimeter back to inch?
Use the reverse factor: 1 millimeter equals 0.0393700787 inches. You can also use the swap control in the converter above to flip the direction instantly.
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.
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.
Is the inch to millimeter 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 inch still used today?
United States and United Kingdom (small measurements, screen sizes, hardware)