Abstract This paper presents a mechanistic model to predict natural separation efficiency. Based on drift-flux model, a new mathematical formulation was developed to estimate natural separation efficiency. The new model considers the existence of the slip velocity not only in the vertical direction, but also in the radial direction. The new and robust model is a function of important variables such as geometric characteristics of the bottomhole, slip velocity at both vertical and radial direction and the gas and liquid flow rates. Available experimental data was used to obtain a new correlation for the bubble drag radius, as a function of gas flow rate. Predictions of the model have been compared and verified against experimental data, showing good agreement.