Mm to inches (mm to in)

One millimeter is exactly 0.0393700787 inches. That is a definition rather than a measurement: the imperial and US customary units were fixed against metric ones by the 1959 international yard and pound agreement, so this factor does not drift and will not be revised.

Worked through: 25 mm x 0.0393700787 = 0.984252 in.

Going the other way, one inch is 25.4 millimeters.

To convert without a calculator, multiply by 0.03937. Rounding that to 0.039 is within 0.94% for most purposes.

Millimeters are where most engineering and machining work happens, and the exact factor matters: 25.4 mm to the inch means a thousandth of an inch is 0.0254 mm, which is why machinists speak in 'thou' and metric shops in hundredths of a millimeter.

Precision is worth matching to the input. A measurement given as 1 mm carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.