Solidity, X (often referred to as packing density) is the volume of solid material in a unit volume of medium. It can also be expressed as the volume fraction of the solids in the medium. In a nonwoven, the solids will include fiber, resin, additives, and any other ingredient that contributes to the solid volume of the structure. Porosity, c = (l - X) is the three-dimensional volume void fraction of the medium. Porosity is not to be confused with air permeability. Permeability as described above is the flow rate of fluid through the medium at a given pressure drop. Both, the solidity X and the porosity c are important properties of filter media that are very evident in the theoretical discussions of Chapter 2. They can be "apparently" calculated frombasis weight and caliper of the medium and the density of its materials,/9 m, as follows: