Inches to Millimeters

American rulers split the inch into halves, quarters, eighths and sixteenths, while metric rulers count whole millimeters. To move between them, turn the fraction into a decimal and then multiply: 3/8 in is 0.375 in, and 0.375 × 25.4 gives 9.525 mm.

The results rarely land on a round number. A quarter inch is 6.35 mm, so a part marked 1/4 in will not match a 6 mm metric part even though the two look alike. When a fit has to be tight, buy the size in the system the drawing uses instead of the nearest match from the other one.

Before 1959 the United States and the United Kingdom defined the inch slightly differently, and the gap mattered in precision work. The international yard and pound agreement of that year fixed the yard at exactly 0.9144 m, which makes the inch exactly 25.4 mm. NIST lists the factor as exact in its guide to SI units (SP 811).

One older unit lingered: the US survey foot, which is a few parts per million longer than the international foot. NIST and NOAA retired it for new work at the end of 2022, so modern drawings and maps use the same inch as everyone else.

Screens, photo prints and paper are often sold in inches even where everything else is metric. A 4 × 6 in photo print is 101.6 × 152.4 mm, and a 6.1 in phone screen measures about 155 mm corner to corner, because screen sizes are taken along the diagonal.

If you are preparing an image for print, the physical size depends on its pixels and its DPI setting. The pixel converter turns pixels into inches or centimeters, and resize image to cm sets an exact printed size for you.