The nanoscale periodic multiphase microstructures evolved via spinodal decomposition assisted phase transformation pathways (PTPs) could potentially improve the mechanical and magnetic properties as reported in multi-principal element alloys (MPEAs) such as Al_0.5NbTa_0.8Ti_1.5V_0.2Zr, TiZrNbTa and Fe₁₅Co₁₅Ni₂₀­Mn₂₀Cu₃₀. However, the design of such microstructurally engineered multi-phase multi-principal element alloys demand a better understanding of spinodal decomposition in multi-component alloys. Thus, we developed a CALPHAD framework based on the existing theories of spinodal decomposition to predict the spinodal boundary and the initial concentration modulations (CMs) during the early stages of spinodal decomposition. We apply this framework to a diverse set of multi-phase MPEAs that have been studied in the literature including the above-mentioned alloys. The MPEA systems are shown to be unstable only to certain concentration modulations which could be further explored to design microstructurally engineered alloys. The developed CALPHAD framework is currently implemented in the latest version of the commercially available CALPHAD software PANDAT and could be utilized to assist in the high throughput microstructurally engineered alloy design strategies.