Collection efficiency of an impactor depends on the collection substrates. Compared to conventional flat substrate, porous substrates increase the collection efficiency. Air flow from the impactor jet can penetrate into the substrate and shift the cutpoint. In this paper Walter John's (John, 1999) simple but accurate derivation for the cutpoint of an impactor is extended to include the effect of flow penetration. Cutpoint shift can be calculated accurately for rectangular nozzle but for round nozzle additional approximations are needed. Results calculated with the simple method agree with the results of more detailed numerical calculations and qualitatively also with experiments. The method can be used to predict the change in the cutpoint when using porous substrates. Method is useful when designing impactors with porous substrates. (C) 2003 Elsevier Ltd. All rights reserved.