The thickness of plastic (plastic gauge) is expressed in microns.
The thickest plastic sheeting (not hard plastic but what is known as flexible plastic sheeting) is HDPE which goes up to 100 mil = 1/1000 inch. HDPE is a form of polyethylene that has a more compact molecule and has a higher density. HDPE comes in thicknesses of 12 mil, 20 mil, 30 mil, 80 mil, and 100 mil. 100 mil is like stacking 3 1/3 credit card on top of each other. A credit card is approximately 30 mil thick.
HDPE has proven itself in industrial, agricultural and municipal projects. It excels in decorative situations as well. This geomembrane is produced from virgin polyethylene resins. It has outstanding chemical resistance, mechanical properties, environmental stress crack resistance, dimensional stability and thermal aging characteristics thus it's known for its strength to density ratio.