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.