1 inch = 25.4 mm
1 in = 25.4 mm · type math like (12*4)+6.5
Inches to millimeters is the everyday conversion for machinists, 3D printers and DIYers ordering metric parts to fit American-sized stock. When a drawing says 3/8 inch and your calipers read millimeters, this is the number you need.
3D printing and laser cutting are a big source of this conversion. Many CAD files from American hobbyists are drawn in inches, but slicer software and laser cutters default to millimeters. Import a 4 inch part without changing units and it appears as a tiny 4 mm part, 25.4 times too small. Always check the import unit setting.
Machinists also use thousandths of an inch (thou) for tolerances. A tolerance of ±0.005 in is ±0.127 mm, while a typical 3D printer's practical accuracy of about 0.1 mm is close to 0.004 in.
Rainfall gauges in the US measure in inches. One inch of rain is 25.4 mm, which on one square meter is 25.4 liters of water.
Wire gauges in the US also relate to inches: 12 AWG copper wire is 0.0808 in thick, 2.05 mm.
To sanity-check, remember that 4 inches is about 100 mm (101.6 exactly) and 12 inches is about 300 mm (304.8 exactly). If you convert 2 inches and get 5 mm, you divided instead of multiplying by 25.4. Digital calipers usually have an in/mm button, which is handy for checking a converted figure against the real part rather than relying on arithmetic alone.
mm = in × 25.4
Reverse: 1 mm = 0.039370079 in.
Convert 3/8 inch to millimeters.
Answer: 3/8 in = 9.525 mm.
| in | mm |
|---|---|
| ¼ in | 6.35 mm |
| ½ in | 12.7 mm |
| ¾ in | 19.05 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 |
| 11 in | 279.4 mm |
| 12 in | 304.8 mm |
| 13 in | 330.2 mm |
| in | 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 |
| 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 |
Nominal 1/2 inch plywood is 12.7 mm on paper; 3/4 inch is 19.05 mm.
A phone marketed as 0.31 inches thick is 7.87 mm.
A 2.5 inch laptop drive is about 63.5 mm wide; a 3.5 inch desktop drive is 101.6 mm wide.
A 1/4 inch hole is 6.35 mm, so an M6 bolt rattles in it and an M7 does not exist in common sizes. Check the thread system too.
Turning 5/16 into 0.31 instead of 0.3125 gives 7.87 mm instead of 7.9375 mm. Keep the fraction exact.
A 2 × 4 is really 1.5 × 3.5 inches, or 38 × 89 mm, not 50.8 × 101.6 mm.
The international inch is exactly 25.4 mm.
Origin. Historically three barleycorns long, the inch was standardised across English-speaking countries in 1959.
Used today. Inches measure US tools, fasteners, tubing and lumber, and fractions like 1/16 and 1/32 are common on tape measures.
A millimeter is 0.001 m, so one inch contains 25.4 of them.
Origin. Defined with the metric system in 1795.
Used today. Millimeters dominate technical drawings, digital calipers, 3D printer settings, bolt sizes (M6, M8) and phone thickness specs.
1.5875 mm, commonly rounded to 1.6 mm.
Those are metric filament sizes; 2.85 mm filament is often called 3 mm. Neither is an inch size, though 1.75 mm is close to 0.069 in.
Pipe sizes are nominal. A 1 inch steel pipe has an outside diameter of 33.4 mm, not 25.4 mm.
Nearly. 3/4 in is 19.05 mm, so a 19 mm socket usually works but may feel snug.
Multiply by 0.0254. A thou (0.001 in) is 0.0254 mm, so 10 thou is 0.254 mm.