Millimeters to Inches Converter

The formula

1 inch equals exactly 25.4 millimeters, so converting the other direction means dividing by 25.4. The tool takes your millimeter value and divides it by 25.4, displaying the result to four decimal places rather than two — millimeter measurements are often precise enough that rounding to hundredths of an inch would lose meaningful detail.

Worked example

A standard sheet of A4 paper is 210 mm wide. Enter 210 and the tool returns 210 ÷ 25.4 = 8.2677 inches — close to, but not exactly, the 8.5-inch width of US letter paper, which is the actual reason A4 and letter-size documents don't print cleanly on each other's paper trays.

Reference sizes

100 mm ÷ 25.4 = 3.9370 in. A 3 mm drill bit converts to 0.1181 in — closest to a US 3/16" bit at 0.1875 in, not an exact match, which is why metric and imperial drill/screw sizes don't interchange cleanly.

Why four decimal places

Millimeter-scale work (drill bits, screw threads, PCB tolerances) often needs precision below 0.01 inch. Four decimal places (ten-thousandths of an inch) keeps that precision intact instead of rounding it away.