This paper presents a numerical study of techniques able to provide the constriction size distribution for granular materials. The cumulative constriction size distribution curve is critical to understand the behaviour of fi lters in earth dams. In this study, we compare three different methods able to provide the constriction size distribution for spherical materials: a DEMDT (Discrete Element Method + Delaunay Tessellation), an analytical method and an inverse method based on numerical experiments of fi ltration tests. The analytical method which is simple of use provides a result close to the DEMTD but needs to be adapted for actual materials. The grading of the fi ne materials used for fi ltration tests in the inverse method seems to play an important role in the derived constriction size distribution. A procedure is proposed for the chose of the grading for the fi ne material on the basis of a parametric study.