The density of plastic is quite variable, as there are a multitude of different types of plastic; typically, plastics have a density from 0.5 to 4.0 g/cm3. On the low end, there are plastics like Polypropylene Uniboard Standard, which has a density of 0.65 g/cm3. On the high end, there are plastics like TFE Tetron B, which has a density of 3.9 g/cm3.
Density is calculated by mass over volume and is measured in grams per centimeters cubed, even in countries that use the imperial measuring system. There are three different types of densities when speaking of plastics: bulk density, solid density and melt density.