Including detailed information of equilibrium phase diagrams (EPhD) is is one of the necessary features in phase transformation simulations. It is important for a simulation to provide a simple but accurate EPhD calculation method which is based on the knowledge of the phase boundary lines in the EPhDs as an mathematical function. A simple, fast and iteration free approximate calculation method has been developed earlier (ESTPHAD – Estimated Phase Diagrams). The input data may be either experimental values, or a data field calculated by CALPHAD, or digitized data from the graphical representation of measured and published EPhDs . By this method important parameters such as the beginning and ending temperatures of solidification or solid state transformations can be determined for each phase. The partition ratios as a function of the concentration can be calculated as well. In this study ESTPHAD is used to estimate the EPhD of binary Fe-Ni system.