An approach for calculating the separation efficiency of uniflow cyclones for the separation of solid particles from gases is proposed. The analytical model is based on an equilibrium orbit concept, similar to that used in the Barth-Muschelknautz model for conventional reverse-flow cyclones, which has been proven to be successful for designing and calculating cyclones in a wide range of industrial applications. The proposed model takes into account the special flow pattern of uniflow cyclones, which differs substantially from that in reverse-flow cyclones. The model provides correct dependencies of the separation efficiency on the main geometry and operation data of low-loaded uniflow cyclones. Applying the calculation method to uniflow cyclones operated in test facilities indicates good agreement with experimental data.