Which louvres are suitable for achieving the required airflow at a given pressure difference?
Known pressure difference over louvre and average air velocity through the louvre
OR pressure difference over the louvre), surface area of louvre and airflow through louvre
Calculated, maximum permissible air resistance of the louvre = suitable louvre types

In case of small dimensions, the real free area of a louvre can deviate significantly from the theoretically calculated value. That is why it is recommended for louvres with less than 6 blades, to provide the louvres larger than calculated or to request additional advice.