1 in = 25.4 mm
| Inch (in) | Millimeter (mm) |
|---|---|
| 0.0625 | 1.5875 |
| 0.125 | 3.175 |
| 0.1875 | 4.7625 |
| 0.25 | 6.35 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 0.625 | 15.875 |
| 0.75 | 19.05 |
| 0.875 | 22.225 |
| 1 | 25.4 |
| 1.5 | 38.1 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 12 | 304.8 |
| 24 | 609.6 |
To convert inches to millimeters, multiply the number of inches by 25.4. There are exactly 25.4 mm in one inch.
1 in = 25.4 mm
Convert 3/8 inch to mm: 0.375 × 25.4 = 9.525 mm
Convert 4.5 inches to mm: 4.5 × 25.4 = 114.3 mm
To convert millimeters to inches, divide the millimeters by 25.4.
\u2022 1 inch (25.4 mm) is the approximate diameter of a US quarter coin.
\u2022 3/8 inch (9.525 mm) is a common drill bit size and socket wrench size.
\u2022 6 inches (152.4 mm) is about the length of a dollar bill.
\u2022 12 inches (304.8 mm) is one foot — roughly the length of a standard ruler.
Converting inches to millimeters is essential for anyone working with precision measurements in engineering, woodworking, machining, or hardware. Since 1 inch equals exactly 25.4 mm, the conversion is precise but the numbers can be cumbersome — making a calculator tool invaluable.
Screws, bolts, nuts, and nails come in both imperial (inches) and metric (mm) sizes. A 1/4-inch bolt is 6.35 mm, a 5/16-inch is 7.9375 mm, and a 3/8-inch is 9.525 mm. When working with imported machinery or replacing parts, having the exact mm equivalent prevents costly mistakes.
American woodworkers measure in fractions of inches (1/4”, 3/8”, 1/2”), while European plans and machinery often use millimeters. Table saw blades, router bits, and plywood thicknesses are specified differently depending on origin. A “3/4-inch” plywood sheet is 19.05 mm, but metric plywood is often 18 mm — a difference that matters for joinery.
CNC machines, lathes, and milling machines accept programs in either inches or millimeters. Converting between them accurately is critical for producing parts that fit together. A tolerance of ±0.001 inches is ±0.0254 mm — knowing this conversion ensures your specifications translate correctly between measurement systems.
One inch equals exactly 25.4 millimeters.
First convert the fraction to a decimal (e.g., 3/8 = 0.375), then multiply by 25.4. So 3/8 inch = 0.375 × 25.4 = 9.525 mm.
1/4 inch equals exactly 6.35 millimeters.
1/2 inch equals exactly 12.7 millimeters.
In 1959, the international yard and pound agreement defined the inch as exactly 25.4 millimeters. This is an exact definition, not an approximation.