Semi-open wall-flow channels are conventional filter channels with missing plugs either in the frontal or rear side. This configuration has lately become an interesting modelling issue with respect to partial filters designed for the emerging markets and as a special case of wall-flow filters, that have suffered damage due to regeneration. This paper presents the development of approximate analytical expressions for the estimation of pressure drop and filtration efficiency of semi-open channels. In order to facilitate the derivation, realistic assumptions are employed and the results are extensively validated using a commercial simulation software, able to solve the differential equations system of the 1D channel model. The validation covers important design parameters and operational conditions, namely filter permeability, length, mass flow rate and soot loading. It is shown that the analytical expressions can predict all trends and moreover offer clear physical explanations. The pressure drop calculation does not exceed 6% of the numerical prediction, while the filtration efficiency absolute error lies within 6% except for specific cases. Finally, a partially failed DPF is simulated using the developed expressions and a commercial CFD package in order to estimate the failure effect on filtration and pressure drop performance. In this way, the usability of the proposed modelling approach is demonstrated.