The transport phenomena in packed beds are sensitive to porosity variations. However, empirical correlations in the literature are not in agreement regarding the prediction of the mean porosity for packed beds, resulting in challenges for reactor design. In this study, the mean porosity variations in a densely packed bed with 1.2 < D/d < 7.1 are investigated experimentally. Moreover, a layer-stacking model is developed to explore the mean porosity variations for different D/d ratios. It is observed that analytical expressions obtained by the model can effectively track the experimental data for D/d < 3. When the D/d ratio increases further, changes in the mean porosity values can be interpreted by the model. Furthermore, it is confirmed that the mean porosity variations exhibit several inherent fluctuations as the D/d ratios vary, which may contribute to the significant differences between values predicted by the correlations in the literature. This significantly reduces the applicability of empirical correlations in the rigorous analysis of packed beds. Finally, the significance of the model is discussed with respect to the prediction of the pressure drop in the packed bed. This study can provide new perspectives to model mean porosity variations in packed beds, which is also useful for the evaluation of the hydraulic performance. (C) 2019 Elsevier B.V. All rights reserved.