Inches to Millimeters Converter
Inches to millimeters is the engineer's and machinist's conversion. When precision drawings, fasteners, drill bits, or machined parts cross between imperial design files and metric workshops, the inch must be translated into the millimeter grid that most of the world manufactures in.
Looking for the reverse? Millimeters to Inches converter →
How to Convert Inches to Millimeters
To convert inches to millimeters, multiply the number of inches by 25.4.
Worked Examples
- 1 inch = 1 × 25.4 = 25.4 mm
- 5 inches = 5 × 25.4 = 127 mm
- 12 inches = 12 × 25.4 = 304.8 mm
- 36 inches = 36 × 25.4 = 914.4 mm
Why Convert Inches to Millimeters?
- Converting US-spec bolt and screw diameters (1/4", 3/8", 1/2") to metric drill sizes for a metric workshop
- Translating drawing dimensions from inch-based CAD files into the millimeter-based tool paths most CNC machines expect
- Sizing wrench, socket, and Allen key sets when working across imperial and metric fastener inventories
- Reading watch lug widths, camera filter threads, and other small-product specs that may be quoted in either unit
- Converting plumbing pipe sizes between nominal inches and millimeter outer diameters
Inches to Millimeters Conversion Table
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.01 in | 0.254 mm |
| 0.1 in | 2.54 mm |
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 0.75 in | 19.05 mm |
| 1 in | 25.4 mm |
| 1.5 in | 38.1 mm |
| 2 in | 50.8 mm |
| 2.5 in | 63.5 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 7 in | 177.8 mm |
| 8 in | 203.2 mm |
| 9 in | 228.6 mm |
| 10 in | 254 mm |
| 11 in | 279.4 mm |
| 12 in | 304.8 mm |
| 13 in | 330.2 mm |
| 14 in | 355.6 mm |
| 15 in | 381 mm |
| 16 in | 406.4 mm |
| 17 in | 431.8 mm |
| 18 in | 457.2 mm |
| 19 in | 482.6 mm |
| 20 in | 508 mm |
| 24 in | 609.6 mm |
| 30 in | 762 mm |
| 36 in | 914.4 mm |
| 48 in | 1219.2 mm |
| 60 in | 1524 mm |
| 72 in | 1828.8 mm |
| 84 in | 2133.6 mm |
| 96 in | 2438.4 mm |
| 100 in | 2540 mm |
| 120 in | 3048 mm |
| 144 in | 3657.6 mm |
| 200 in | 5080 mm |
| 300 in | 7620 mm |
| 500 in | 12700 mm |
| 1000 in | 25400 mm |
Quick Reference: Inches to Millimeters
The most-searched conversion values for this pair, summarised for quick lookup:
| Inch | Millimeters |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
What Is an Inch?
An inch is a unit of length equal to exactly 25.4 millimeters, defined in 1959 as part of the international yard and pound agreement.
The inch traces back to early English measurement, originally tied to the width of a human thumb or three barleycorns laid end-to-end. After centuries of regional variation, the international inch was standardized in 1959 by an agreement among English-speaking countries to exactly 25.4 millimeters, replacing the slightly different US survey inch for most purposes.
In everyday use: Inches are the standard unit for screen sizes (a 15-inch laptop), TV diagonals, paper formats in the US, plumbing pipe diameters, and clothing dimensions like waistlines and shoe insoles. They remain dominant in US construction, woodworking, and any industry that grew around imperial drawings.
The inch symbol is in.
What Is a Millimeter?
A millimeter is a metric unit of length equal to one thousandth (1/1000) of a meter. It is the smallest commonly used metric unit for length.
The millimeter shares its origin with the meter, dating to the 1790s French definition of the metric system. As precision manufacturing matured in the 19th and 20th centuries, the millimeter became the preferred unit for engineering drawings outside the United States — fine enough for tight tolerances, but still readable without specialised tooling.
In everyday use: Millimeters dominate mechanical engineering, machining, and CAD drawings, as well as ammunition calibers (9 mm) and the thickness of materials like glass, sheet metal, or smartphone bezels. Camera lens focal lengths, watch case diameters, and most consumer-electronics dimensions are also published in millimeters.
The millimeter symbol is mm.
Precision and Accuracy
Exactly 25.4 millimeters per inch. The conversion is exact by international definition (1959), so even the long decimals (5 inches = 127 mm) carry no rounding error.
For most everyday purposes — recipes, room sizing, shopping — four decimal places of precision are more than enough. Engineering and scientific work may require additional digits or scientific notation for very small or very large results.
Common Inches to Millimeters Conversions
References
- National Institute of Standards and Technology (NIST) — SI Units: Length
- International Bureau of Weights and Measures (BIPM) — The International System of Units (SI)