How many millimeters are in one inch?

By definition there are exactly 25.4 millimeters per inch.

Effective July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be exactly 0.9144 meters. Consequently, the international inch is defined to be equal to exactly 25.4 millimeters.

To convert millimeters into inches, divide millimeters by 25.4.

To convert inches into millimeters, multiply inches by 25.4.
25.4 millimeters ==
25,4 mm
25.4 mm = 1 inch

Algebraic Steps / Dimensional Analysis Formula1 in*

2.54 cm

1 in

*

10 mm

1 cm

=

25.4 mm
There are 25.4 millimeters in 1 inch.

But, if you are measuring Millers, I'd say the average male miller is .01388889 Millers per inch, or 72 inches per Miller.
There are 25.4 millimetres in one inch.