Source unit
Inch in
An inch is a length unit used in US customary and Imperial measurement.
One inch equals exactly 25.4 millimeters.
Inch to Millimeter Converter
Reverse conversion: Millimeter to InchResult
Inch to Millimeter
Exact1 in=25.4 mm
How to convert inches to millimeters
Conversion table
Practical and common reference values
| Inches (in) | Millimeters (mm) | Copy converted value |
|---|---|---|
| 1/32 (0.03125) | 0.79375 | |
| 1/16 (0.0625) | 1.5875 | |
| ⅛ (0.125) | 3.175 | |
| ¼ (0.25) | 6.35 | |
| ½ (0.5) | 12.7 | |
| 1 | 25.4 | |
| 2 | 50.8 | |
| 4 | 101.6 | |
| 12 | 304.8 |
About Inches and Millimeters
Inches are imperial and US customary length units; millimeters provide fine metric resolution. One international inch equals exactly 25.4 millimeters.
Inch as the source, Millimeter as the result
Use this direction when an inch or fractional-inch dimension must be entered into a metric drawing, matched to a millimeter component, or compared with a metric tolerance.
Result unit
Millimeter mm
Millimeter is a unit used to measure length.
One millimeter equals 0.001 meters.
Where inch-to-millimeter conversion is useful
Inch-to-millimeter conversion is common for fasteners, drill sizes, tools, tubing, sheet materials, manufacturing drawings, and product specifications that mix customary and metric dimensions.
A finite exact millimeter factor
The international inch is exactly 25.4 millimeters, so the conversion factor is both exact and finite. Result precision should nevertheless reflect how precisely the inch value was measured or specified.
Interpret fractional inches before calculating
A fraction such as 3/8 inch represents 0.375 inch, not 0.38 inch. Preserve the fraction or convert it accurately before applying 25.4, especially when checking hardware sizes and machining tolerances.
How this conversion is verified
The international-inch definition fixes one inch at exactly 25.4 millimeters. The converter applies that definition directly to the live result, fractional reference values, and generated table.
Standards used for this conversion
For mental checking, 1/2 inch is 12.7 millimeters and 1/4 inch is 6.35 millimeters. Use the converter for less familiar fractions.
The factor 25.4 applies to linear inches. Square inches and cubic inches use different area and volume relationships.
Frequently asked questions
6 answers for this conversion
- How do I convert inches to millimeters?
- Multiply the inch value by exactly 25.4. For example, 2 inches equals exactly 50.8 millimeters.
- How many millimeters are in one inch?
- One international inch equals exactly 25.4 millimeters.
- How do I convert a fractional inch to millimeters?
- Evaluate the fraction as an inch value, then multiply by 25.4. For example, 3/8 inch is 0.375 inch and equals exactly 9.525 millimeters.
- Is the inch-to-millimeter conversion exact?
- Yes. The international inch is defined as exactly 25.4 millimeters, so the unit relationship introduces no approximation.
- Should I round an inch-to-millimeter result?
- Round only when the receiving specification requires it, and do not show more measurement precision than the original inch value supports.
- Does 0.375 inch mean the same as 3/8 inch?
- Yes. Both represent the same length, and both produce 9.525 millimeters under the exact international-inch definition.
Explore length conversions
30 compatible units