Inches to Millimeters Converter (in to mm)
1 in = 25.4 mm exactly. Convert decimal or fractional inches to millimeters for drill bits, sockets, pipe and machined parts, with an mm chart.
mm = in × 25.4
Inch (in): The inch is exactly 2.54 centimetres, a value fixed by the 1959 international yard and pound agreement. It is used in the US and UK for screen sizes, tools, pipes and body measurements.
Millimeter (mm): The millimetre is one thousandth of a metre (0.001 m). There are 10 millimetres in a centimetre, and exactly 25.4 millimetres in an international inch.
| Inch (in) | Millimeter (mm) |
|---|---|
| 1 | 25.4 |
| 5 | 127 |
| 10 | 254 |
| 25 | 635 |
| 50 | 1270 |
| 100 | 2540 |
Drill bits, sockets, pipe threads and hardware sold in the US are still sized in inches, very often as fractions such as 5/16 or 3/4, while machine drawings, 3D-printing software and most spec sheets work in millimeters. One inch is exactly 25.4 mm, so a fractional size converts without any rounding at all: a sixteenth of an inch is exactly 1.5875 mm.
The main chart steps through the sixteenths and eighths of an inch found on a tape measure, written as decimals (0.1875 is three-sixteenths). The everyday table lists hardware sizes, and the second chart turns metric values back into inches when you need the nearest inch tool.
How to use this tool
- Type a value in the Inch box to see it in millimeters, or type in the Millimeter box to convert the other way.
- Press ⇄ to swap the two units, or Load example for a typical value.
- Check the charts below for common values; for other units, open the Length converter.
Formula
mm = in × 25.4
in = mm ÷ 25.4
Both factors come from exact definitions: 1 millimeter = 0.0393700787402 inches and 1 inch = 25.4 millimeters (shown to 12 significant digits).
Example
Two conversions worked with the same factors the converter uses:
0.375 in × 25.4 = 9.525 mm
6 mm × 0.0393700787402 ≈ 0.23622 in
Inch to Millimeter chart
Common values from inches to millimeters, rounded to four decimal places.
| Inch (in) | Millimeter (mm) |
|---|---|
| 0.0625 | 1.5875 |
| 0.1875 | 4.7625 |
| 0.25 | 6.35 |
| 0.3125 | 7.9375 |
| 0.375 | 9.525 |
| 0.4375 | 11.1125 |
| 0.5 | 12.7 |
| 0.5625 | 14.2875 |
| 0.625 | 15.875 |
| 0.6875 | 17.4625 |
| 0.75 | 19.05 |
| 0.8125 | 20.6375 |
| 0.875 | 22.225 |
| 0.9375 | 23.8125 |
| 1 | 25.4 |
| 1.125 | 28.575 |
| 1.375 | 34.925 |
| 1.625 | 41.275 |
| 1.875 | 47.625 |
| 2.25 | 57.15 |
| 2.75 | 69.85 |
| 3.5 | 88.9 |
| 5 | 127 |
| 8 | 203.2 |
| 10 | 254 |
| 24 | 609.6 |
Millimeter to Inch chart
The reverse direction for values people often look up.
| Millimeter (mm) | Inch (in) |
|---|---|
| 0.1 | 0.0039 |
| 0.2 | 0.0079 |
| 0.25 | 0.0098 |
| 0.8 | 0.0315 |
| 1.2 | 0.0472 |
| 1.6 | 0.063 |
| 2.5 | 0.0984 |
| 3.2 | 0.126 |
| 4.5 | 0.1772 |
| 7 | 0.2756 |
| 9 | 0.3543 |
| 11 | 0.4331 |
| 14 | 0.5512 |
| 16 | 0.6299 |
| 18 | 0.7087 |
| 24 | 0.9449 |
| 28 | 1.1024 |
| 32 | 1.2598 |
Inch sizes on tools and hardware
| Item | Inch (in) | Millimeter (mm) |
|---|---|---|
| Hex shank of a standard screwdriver bit | 0.25 | 6.35 |
| Socket wrench drive, common mid size | 0.375 | 9.525 |
| Socket wrench drive, heavy-duty size | 0.5 | 12.7 |
| Standard drywall sheet thickness | 0.5 | 12.7 |
| Garden hose, common inner diameter | 0.625 | 15.875 |
| Golf ball, minimum diameter (Rules of Golf) | 1.68 | 42.672 |
| Bank card, long side | 3.37 | 85.598 |
| Bank card, short side | 2.125 | 53.975 |
Quick mental estimate
Multiply by 25, then add 0.4 mm for each inch: 3 in → 75 + 1.2 = 76.2 mm, which is exact because 25.4 is 25 plus 0.4. Stopping after the first step leaves the answer about 1.6% short.
Use the converter or the charts when the exact figure matters.
The two units
Inch (in): The inch is exactly 2.54 centimetres, a value fixed by the 1959 international yard and pound agreement. It is used in the US and UK for screen sizes, tools, pipes and body measurements.
Millimeter (mm): The millimetre is one thousandth of a metre (0.001 m). There are 10 millimetres in a centimetre, and exactly 25.4 millimetres in an international inch.
Frequently asked questions
How do I convert 5/16 inch to mm?
Divide 5 by 16 to get 0.3125, then multiply by 25.4. The result is exactly 7.9375 mm, which is why a 5/16-inch bolt is close to, but not the same as, an 8 mm one.
What is the difference between 1/4 inch and 6 mm?
A quarter inch is 6.35 mm, so it is 0.35 mm larger than 6 mm. That is enough for a 6 mm dowel to wobble in a quarter-inch hole, and for a quarter-inch router bit shank not to fit a 6 mm collet.
Are inch-sized pipes really that diameter?
No. Nominal pipe sizes are trade names: a 1/2-inch steel pipe has an outside diameter of 0.840 in (21.3 mm). Convert the measured diameter, not the nominal size, when matching metric fittings.
How precise is the result?
The converter uses the exact definitions above with browser floating-point arithmetic and shows up to 12 significant digits. The charts round to four decimal places for reading.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/inch-to-millimeter-converter/
Cite or embed this tool
kakaoCalc. (2026, November 8). Inches to Millimeters Converter (in to mm). https://kakao.mobile/tools/inch-to-millimeter-converter/
“Inches to Millimeters Converter (in to mm).” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/inch-to-millimeter-converter/.
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