Inches to Millimeters

Engineers and machinists in the US measure in thousandths of an inch, called thou or mils. One thou is 0.0254 mm, or 25.4 micrometers, so a tolerance of ±0.005 in is ±0.127 mm. Because 25.4 is an exact factor, these conversions never add rounding of their own; you only round at the end to the precision your instrument can read.

Fractions convert the same way once they are decimals: 1/8 in is 3.175 mm, 3/8 in is 9.525 mm, 5/8 in is 15.875 mm. The quarter-inch tripod thread on cameras, written 1/4-20, has a 6.35 mm diameter and 20 threads per inch, so a 6 mm metric bolt will not hold in it.

Tools show how close sizes can be without being equal. A 1/2 in wrench is 12.7 mm and a 13 mm wrench is only 0.3 mm larger, so it often fits but can round off a tight bolt. A 3/4 in socket is 19.05 mm against a 19 mm metric one, a near-perfect match, while 5/16 in at 7.94 mm sits awkwardly between 7 and 8 mm.

Nominal sizes are a separate trap. A North American 2 × 4 stud is actually 1.5 × 3.5 in, or 38.1 × 88.9 mm, after drying and planing. A 1-inch steel pipe has an outside diameter of 1.315 in, 33.4 mm, because the name refers to a historic bore size. Measure the real part before converting a label. The 3.5-inch floppy disk is the reverse case: it was designed as a 90 mm metric part and marketed in inches.

Millimeters are much smaller than inches, so the result must always be larger than the input, by a factor of about 25. If 4 in comes out as 10.16, you multiplied by 2.54 and got centimeters; move the decimal one place. For the reverse direction, go to millimeters to inches, and for every other length use the unit converter.