A mil is a U.S. unit of measurement that is also referred to as a thou because it is equivalent to one-thousandth of an inch. This unit of measurement is often used to measure the thickness of sheets like paper, foil, plastic and sheet metal, as well as the manufacturing of automobile engine parts. Gauge is used for several measurements, too, including gun bores, diameter of wire and the thickness of sheets. It is in the measurement of sheet materials where there exists an overlap in definition as well as a direct correlation.
Record your number of mils or enter the number into a calculator. For example, suppose a thickness is 1 mil.
Multiply your number of mils by 100. This is the conversion factor.
Record your number in gauge. For example, 1 mil times 100 is 100 gauge.
TL;DR (Too Long; Didn't Read)
If you wish to convert mil to gauge, multiply by 100. If you wish to convert gauge to mil, divide the gauge measurement by 100.
Mil is not the same as micron, or micrometer. To convert mil to micron, the conversion factor used is 25.4. For example, 2 mil equals 50.8 microns.