A theoretical model based on continuum mechanics theory is developed for predicting the flowrate of bulk solids from mass flow bins with conical hoppers. Close agreement between theoretical predictions and experimental results is obtained. Both theoretical predictions and experimental results indicate that the flowrate of free flowing bulk solids increases rapidly at first and then more gradually as the bulk solid permeability constant increases. This dependence is reinforced by a sensitivity analysis which verifies that the air pressure gradient at the hopper outlet generated during flow has significant effect on the flowrate. The use of permeability enables the theory developed to be applied in describing the flow behaviour of both coarse and fine particle mixtures as well as size-distributed bulk solids. The use of consolidation-related bulk density and permeability relations enables the theoretical model to be applied to both compressible and incompressible materials. Results of both theoretical and experimental work relating to the effect of material surcharge level on th significant. © 1992.