2 inch professional hdpe plate for Seawater desalination 20mm Durable hdpe pad for Seawater desalination The most mmon size in the thickness rating for plastic sheeting is 6 mil. This is 6-thousandths of an inch, or 0.006 inch. large hdpe plastic sheets 20mm st-HDPE 4×8 polyethylene . HDPE. Density. 0.96-1 g/cm3. Shore Hardness. 55-70 A.