The description of the mixing characteristics is of fundamental interest for modeling mass transport in fixed-bed reactors. A method based on Lagrangian parcel tracking is presented to determine the axial dispersion coefficient in the mechanical dispersion regime from particle-resolved computational fluid dynamics (CFD) results. The impact of fluid dynamic entry effects and the necessary averaging length on the axial dispersion coefficient is studied, showing that the often reported dependency on the entry length is almost not existent, but a result of insufficient averaging, caused by to short bed heights. The axial dispersion coefficients are determined for different particle shapes and a novel reactor wall concept. A strong correlation is found between the relative variance of the axial velocity and the axial dispersion coefficient.