Paper presents evaluation of known from literature equations to determinate flow resistances for aerosol flowing through porous filtering layers. Attention was paid on high dependance of layer porosity ε and thickness of fibres on value on drag coefficient k analysis of drag coefficient were made in equations 6-16 and its big variations even for small changes ε and a were shown. On picture 1-3 dependance showing influence ε, a and H on value of flow resistances were presented. Value of calculated pressure droop ΔP when porosity is changed ±1% shows big variations.