Paper consists of a network of pulp or wood pulp fibers of different length, thickness and shape. Due to this fact, the sheet has air gaps that can be partially filled by filler particles. Because of the air gaps, paper is porous. Porosity is the ratio of pore volume to the total volume of the paper. Depending on the type and shape of the pores, they are responsible for gas and liquid transport through the paper, but also for wetting and moisture absorption.

Often, the air permeability of paper is also referred to as porosity. The permeability of paper to gas is defined as the amount of gas (white air) passing through the paper per unit time as a result of a certain pressure difference. This method is standardized according to DIN 53120 or ISO 2965.