The Gibbs energy function of Sr6Co5O15 is calculated by first-principles for use in CALPHAD thermodynamic modeling. An efficient method is employed, using the Debye-Gruneisen model to predict the temperature dependence of the heat capacity and entropy. The equation of state from first-principles and the Debye temperature from harmonic phonon calculations by the supercell approach are taken as input. The effect of using the GGA+U approach on the results is also reported. The properties of Co3O4 are predicted with this method to compare to experiments and quasi-harmonic phonon calculations and are shown to achieve the accuracy necessary for CALPHAD modeling.