A numerical-based fluid mechanics model is proposed to predict churning losses in cylindrical roller bearings, which is caused by the rotation and the translation of the rollers in the bearing cavity. The Computational Fluid Dynamics (CFD) method is conducted to quantify the influence of various factors on churning losses, including operating conditions, roller geometry parameters, and fluid properties. One configuration with several in-line finite-length circular cylinders sandwiched by two flat walls is analyzed in one-phase environment, which represents a simplified approache. The results indicate that the roller orbital speed, the presence of adjacent rollers, and the rings have a significant impact on the churning moments. Finally a formulation is proposed for churning losses prediction in cylindrical roller bearings. This is of particular interest for high-speed applications where churning losses may represent up to 50 % of the total power dissipated.