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The product yields of hydrothermal carbonization of digestate as well as the degree of carbonization of hydrochar are quantified as functions of process parameters by using a severity approach. In contrast to other studies, a logarithmic dependence on process severity was applied. Process severity itself was calculated from temperature, retention time and catalyst concentration. Data gained from batch experiments (190–245 °C, 140–560 min) was used to fit the model parameters. By these models basing on few selected reaction conditions, a wide range of process conditions can be covered and the yields for the solid, liquid and gaseous product phase can be predicted. Moreover, the paper delivers model equations for the prediction of the H/C and O/C ratios for the solid product phase. Such model equations can be used for process optimization and are the foundation for proper LCA calculations. For the first time, the quantitative impact of the difference in reaction conditions on the product phase yield is described and analysed