The void size distribution inside a packed bed is one of the most important parameters affecting the reactor performance. The non-uniform distribution of material and energy flows can be avoided by a reasonable arrangement of the void size distribution. In this work, the void size distributions in a packed bed with structured and random packing were investigated through both theoretical calculations and statistical analysis of experimental results to characterize the bed voids between the particles. In theoretical analysis, the equivalent diameters of voids at different two-dimensional layers in a packed bed with three types of structured packing were first calculated, and then the wax replacement method was adopted to measure the voids inside a randomly packed bed. Different two-dimensional layers of the bed were sliced and photographed for further analysis. The effects of particle size, shape, and packing method were investigated, and the results showed that the relative void size (void size/particle size) was not affected by the particle size. In contrast, the particle shape, which can be represented by particle sphericity, significantly affected the void size distribution. A more flat void size distribution curve was obtained for particles with sphericity close to unity. A new correlation was established based on statistical analysis of the experimental data to account for the void size distribution. This new correlation was validated by predicting the pressure drop of a packed bed and the results of comparison showed good agreement with experimental results. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.