UnitSnapInches to Millimeters
1 in = 25.4 mm
Inches to millimeters matters for machinists, engineers, and DIY builders working with fractional-inch tools or parts alongside metric drawings. Drill bits, pipe threads, bolts, and lumber in the US are often sized in inches. One inch is defined as exactly 25.4 millimeters, so precise conversion is straightforward.
When you need to convert inches to millimeters
- Matching a fractional-inch drill bit to a metric hole specification
- Converting tolerances on an American engineering drawing for a metric shop
- Finding the metric equivalent of a bolt or socket listed in inches
- Checking wheel rim diameters, which are traditionally quoted in inches
How to convert inches to millimeters
mm = in × 25.4
Example: 5 in = 127 mm
Common fractional inch sizes
| Inches (in) | Millimeters (mm) |
|---|---|
| 0.125 in | 3.175 mm |
| 0.25 in | 6.35 mm |
| 0.375 in | 9.525 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 |
| 3 in | 76.2 mm |
Tips for converting inches to millimeters
- Multiply inches by 25 for a rough estimate. It is about 1.6% low, so do not use it for machining or fit-critical parts.
- Convert fractions to decimals first: 1/8 in is 0.125, 1/4 in is 0.25, 3/8 in is 0.375.
- An inch socket and the nearest metric socket are rarely interchangeable. Even a small mismatch can round off a bolt head.
Inches to Millimeters conversion table
| Inches (in) | Millimeters (mm) |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 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 |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 30 in | 762 mm |
| 40 in | 1,016 mm |
| 50 in | 1,270 mm |
| 75 in | 1,905 mm |
| 100 in | 2,540 mm |
| 250 in | 6,350 mm |
| 500 in | 12,700 mm |
| 1,000 in | 25,400 mm |
Fractional inches to millimeters
Tape measures and drill bits use fractions of an inch. You can also type fractions such as 5 1/2 or 3/8 into the converter above.
| Inches (fraction) | Inches (decimal) | Millimeters (mm) |
|---|---|---|
| 1/16 in | 0.0625 in | 1.5875 mm |
| 1/8 in | 0.125 in | 3.175 mm |
| 3/16 in | 0.1875 in | 4.7625 mm |
| 1/4 in | 0.25 in | 6.35 mm |
| 5/16 in | 0.3125 in | 7.9375 mm |
| 3/8 in | 0.375 in | 9.525 mm |
| 7/16 in | 0.4375 in | 11.1125 mm |
| 1/2 in | 0.5 in | 12.7 mm |
| 9/16 in | 0.5625 in | 14.2875 mm |
| 5/8 in | 0.625 in | 15.875 mm |
| 11/16 in | 0.6875 in | 17.4625 mm |
| 3/4 in | 0.75 in | 19.05 mm |
| 13/16 in | 0.8125 in | 20.6375 mm |
| 7/8 in | 0.875 in | 22.225 mm |
| 15/16 in | 0.9375 in | 23.8125 mm |
| 1 in | 1 in | 25.4 mm |
About the units
Inches: The inch is an imperial and US customary unit of length, defined since 1959 as exactly 2.54 centimeters. It is used for screen sizes, clothing and hardware in the US and UK.
Millimeters: The millimeter is a metric unit of length equal to one thousandth of a meter, used for precise measurements in engineering, manufacturing and rainfall.
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Frequently asked questions
How many millimeters are in 1 inch?
1 inch equals 25.4 millimeters.
How many inches are in 1 millimeter?
1 millimeter equals 0.03937008 inches.
How do I convert inches to millimeters?
Use the formula mm = in × 25.4.
What is 10 inches in millimeters?
10 inches equals 254 millimeters.
Related conversions
Factors: NIST SP 811 · SI Brochure · NIST Handbook 44 — Updated