A model for the calculation of liquid-liquid interfacial energies is presented. It is based on the assumption that the interface can be treated as a separate thermodynamic phase. Its derivation has been performed in an analogous way as the derivation of the Butler equation for the surface tension of liquid alloys. It requires as input parameters the excess free energy and the compositions of the bulk phases as functions of temperature. In addition, it also requires the partial molar volumes of the components. Comparison with existing experimental data for Al-Pb, Al-In, and Cu-Co in a non-equilibrium state shows very good agreements. For Al-Bi, the experimental data are either over or underestimated by a factor of a parts per thousand 1.7, depending on which of the two thermodynamic assessments is used. For the Al-based systems, the calculated Al-mole fraction in the interface layer is close to the arithmetic average of the Al-mole fractions of the bulk phases.