The methods for determination of freezing point include cooling curve cryoscopy and air humidity cryoscopy. These methods are usually influenced by many factors, such as instrumentation, environment and operators. Despite the numerous experimental methods, precise freezing point values are challenging to obtain due to time-consuming procedures, limited sample size and extensive manual work. In this study, a semi-empirical hydration model (SEHM) was developed to calculate freezing point of NaH₂PO₄-K₂HPO₄ mixed electrolyte solution. According to SEHM, the water activity of mixed electrolyte solution was calculated by molality of solutes. Then the freezing point of solution was calculated. The calculated results were compared with those obtained by cooling curve cryoscopy and air humidity cryoscopy. The results indicate that the SEHM calculations are comparable to the measurements of cooling curve cryoscopy and air humidity cryoscopy.