The magnetic properties of lanthanides are currently of great interest. Especially, the discovery that mononuclear complexes of rare earth (RE) ions can exhibit magnetic anisotropy and can act as single molecular magnets. Heretofore, the magnetic susceptibility of RE ions has often been estimated, using HUND's empirical formula. In this work, we propose a non-empirical calculation of magnetic susceptibilities of Ln(III) ions based on Van Vleck's formula, that accounts for all excited states within the f(n) manifold. The values obtained with Van Vleck's formula are much better than the ones with HUND's formula and are in good agreement with the experiments. (C) 2013 Elsevier B.V. All rights reserved.