The product yields of hydrothermal carbonization of wheat straw as well as the degree of carbonization are quantified as functions of process parameters by using a severity approach. The process severity was calculated from temperature, retention time, and catalyst concentration. Data gained from batch experiments (190–245 °C,150–570 min) were 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, solved organic, 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 for valid LCA calculations.