Bendtsen air permeability is the ability of paper or board to allow an air flow to pass through it at a given pressure differential. Bendtsen air permeability is determined according to ISO 5636-3, measuring the air flow rate in ml/min through a 10 cm² sample at a pressure differential of 1.47 kPa.

5 measurements each with air flow from top to bottom and 5 measurements with reverse air flow direction are carried out. An average value is calculated from the individual values. If the values of the upper and lower side show significant deviations, the test must be repeated and an average value per paper side must be formed. The air permeability is given in µm / Pa*s.