Anisotropic broadening correction in X-ray powder diffraction by an ellipsoidal formula is applied on samples with nanosized crystals. Two cases of minerals with largely anisotropic crystallite shapes are presented. The properly applied formalism not only improves the fitting of the theoretical and observed diffraction diagrams but also gives direct information about realistic crystallite shapes and sizes. The approach is demonstrated using the Rietveld refinement program TOPAS and it is easily adaptable to other similar software.