Surprising to many, a “mil” varies greatly from a “millimeter.” In addition to sounding nearly identical, both measure the thicknesses of materials in the manufacturing industry. But when talking about decals, the unit “mil” is much more common, equivalent to one-thousandth of an inch, or 0.001 inch. A millimeter is almost 40 times thicker than that, or approximately 0.0394 inch. So despite the similar-sounding names, confusing the two is worse than mixing up thin crust and deep dish pizza.
Check out the full blog post here: gographicsoutput.com/blog/material-thicknesses-mils-vs-millimeters/