Until now all these models have been designed and used with an average fibre diameter. This paper presents a new development taking the fibre size distribution into account for the calculations of filter efficiencies. Two different approaches were used to calculate the permeation of the filter media: the first one decomposes the filter media into n separate slices composed of fibres of a single diameter d(i) arranged in a parallel array. The second one considers the same slices arranged in series. The influence of the filter fibre size distribution standard deviation is investigated maintaining either the mean arithmetic diameter or the mean geometric diameter of the filter fibres. This mathematical development IS pplied to a recent efficiency model and compared with experimental data for three different filter media.