The skin thickness of 4 mm coroplast is around 10 mils thick. The next size up, 6 mm coroplast is around 25 mils thick. Coroplast also comes with different size corrugations. Some will be 1/8 inch square and some will be 1/8 x 3/16 inch. I like the 3/16 better and it weighs a little less. The coroplast with 1/8-inch square corrugations is a little more dense and is stronger. Which type you receive depends on where you order it. If you ask about this when you order your coroplast nobody seems to know what you are talking about when you ask about the size of the corrugations.