This study evaluated the performance of two bottom-spray coaters and the effect of pellet-size variability on coating uniformity. A conventional Wurster chamber was used for the first group of trials, and a Wurster chamber with a novel swirl-flow generator design was used for the second. The results confirmed that when using a conventional Wurster coating chamber, pellets with a smaller diameter receive significantly less coating material compared to those with larger diameters. The swirl generator-equipped Wurster chamber achieved close to uniform coating thickness regardless of pellet size. The ratio (M(S)) of the mass of dye deposited in the coating layer to pellet surface area indicates that coating was much more evenly distributed using the swirl-flow coater. Coating thickness was also analyzed using SEM micrographs and the results were in close agreement with the M(S) factor values. Inter-particle coating mass variation was also lower in case of swirl-flow coater. The results of this study show that a swirl-flow coater is suitable for coating particles of variable size. They also showed an improvement in coating process yield when using the swirl-flow coater.