A multi-scale model is presented for predicting the magnitude and rate of powder blending in blenders. The model combines diffusion coefficient correlations from small-scale discrete element method (DEM) simulations with advective flow field information from blender finite element method simulations. The multi-scale model predictions closely match results from large-scale DEM-only simulations for a rotating drum, but take only a hours to compute as opposed to taking days of computation time for the DEM-only simulations. The multi-scale model scales much more efficiently to industrial-scale blenders than DEM-only simulations.