When you need this conversion
Machining, woodworking, 3D printing, electronics and any product with imperial specs. CAD software, 3D printers and most parts catalogs outside the US work in millimeters.
Fractions to mm
Convert the fraction to a decimal, then multiply by 25.4:
- 1/16 in = 1.5875 mm
- 1/8 in = 3.175 mm
- 3/16 in = 4.7625 mm
- 1/4 in = 6.35 mm
- 5/16 in = 7.9375 mm
- 3/8 in = 9.525 mm
- 1/2 in = 12.7 mm
- 3/4 in = 19.05 mm
Thousandths of an inch
Machinists work in thousandths (“thou” or “mil”): 0.001 in = 0.0254 mm, so 10 thou is 0.254 mm.
Common mistakes
Treating nominal sizes as real sizes. Lumber (a 2×4 is 1.5 × 3.5 in) and pipe sizes are names, not measurements.
Rounding too early. 3/16 in is 4.7625 mm; rounding to 4.8 is fine for wood, not for bearings.
For the reverse, use mm to inches.