Thickness is a measure of the distance between the opposite faces of an object. It is usually measured in millimeters (mm) or inches (in).

Density is a measure of the amount of matter in a given volume of a substance. It is usually measured in kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cm3).

The thickness and density of an object are not always related. For example, a piece of paper can be very thin but have a high density, while a piece of styrofoam can be very thick but have a low density.