The total efficiency of gravity settlers and cyclones is usually calculated by summing the product of the grade efficiency and the weight fraction of particles over a series of particle size intervals. The accuracy of this method strongly depends on the number of the particle size intervals and the width of the individual intervals. Moreover, the design of these devices is based on specifying a required efficiency for a given size of particles. However, since these collectors are usually used for precleaning the dust-laden gas so as to reduce the dust loading before the gas enters a subsequent high-efficiency collector, one needs to know what total efficiency must be achieved. Therefore, the present paper uses a simplified model to predict the total efficiency of these devices based on the assumption of a normal size distribution. It is further shown that the equations developed assuming a normal size distribution can be extended for other particle size distributions.