Air permeability according to Gurley refers to the ability of paper or board to allow an air stream to pass through at a given pressure differential. Gurley air permeability is determined according to ISO 5636-5 using a Gurley densometer.

Here, the time is measured that a defined volume of air needs to flow through the sample. Five measurements each are carried out with air flowing from the top to the bottom and five measurements with air flowing in the opposite direction. An average value is calculated from the individual values. If the values of the top and bottom sides show significant deviations, the test must be repeated and an average value must be calculated for each side of the paper.

The air permeability is given in µm / Pa*s.