The unusual trend of pressure loss relative to that estimated on the basis of a homogeneous bed - i.e. increasing with increasing of wall effects in the viscous flow regime and decreasing with increasing wall effects in the inertial flow regime - is well captured by the simple model, which also yields quantitative results in good accord with the available data for the viscous regime. In spite of its extreme simplicity, it appears to be the first model able to fully accommodate this unusual pattern of behaviour. Its application could not be more simple, involving as it does the application of the standard Ergun relation to the bulk region of the bed in which the void fraction represents that which would obtain in a large diameter bed and the fluid flux is obtained as a function of D/d from the average value U. It therefore circumvents problems of uncertainties with regard to conditions in the wall region, which other approaches to this problem have tended to focus on.