"Mil thickness" refers to a U.S. engineering unit of measurement that is equal to 0.001 inch. Engineers and manufacturers use mil to rate the thickness of various items, including plastic, paper, sheet metal and aluminum foil.
The most common size of plastic sheeting is 6 mil, which is 0.006 inch. Plastic sheeting also comes in 1 mil, 2 mil, 3 mil, 6 mil and 10 mil. Thick, heavy plastic is 12 mil to 80 mil. Copper sheets come in varying thicknesses, including 1 mil, 1.4 mil, 3 mil, 5 mil and 10 mil. Heavier copper sheets are 16 mil and 22 mil. Standard-duty aluminum foil is between 0.4 mil and 0.7 mil or 0.0004 inch and 0.0007 inch. Heavy-duty aluminum foil is 0.8 mil to 1 mil.