The opening size of the discharge outlet and the slope of a conical hopper call be determined using the well-known theoretical method of Jenike. This method enables designing the hopper almost so precisely as to permit mass flow powder discharge (without stagnant zone). On the other hand, a flowability index is Customarily used as a measure of the ease of powder flow. From a practical point of view, it should be desirable to predict whether a given powder will form arches or bridges that will prevent its mass flow discharge from the hopper. In order to obtain such a predictive tool, we have studied the relationships, for 90 powders, between (1) the mass flow hopper outlet size and flowability index, and (2) the mass flow hopper slope and flowability index. In both cases, there is a considerable scattering of experimental data, but limiting functional relations have been obtained that allow practical determination of safety values for mass flow hopper outlet size and slope as a function of the flowability index.