A model was developed to predict the relative velocity necessary for the calculation of the settling velocity in fluid-particle sedimentation systems, both binary and polydisperse suspensions. The model obtained was tested against the experimental data obtained from a countercurrent flow system and was found to give an excellent fit. The Reynolds number ranges between 4 and 1300. The model predictions were compared with those of other published models. The model predictions have shown an improvement and do not have the limitations present in the other models.