In recent years, hydrothermal carbonization (HTC) has been introduced as an attractive method for converting biomass into value-added products. The complex reaction chemistry and variable composition of biomass have, however, been barriers to find general equations for describing the HTC process. The goal of this study is to establish a link between the composition of biomass and the expected hydrochar from HTC. Based on the experimental design found from response surface methodology, the biomass components, namely pure cellulose, hemicellulose, and lignin, were submitted in different combinations into 39 HTC experiments with severity factors (SFs) of 3.83, 5.01, and 6.19. Using the experimental data, an attempt was then made to predict the mass yield, higher heating value (HHV), carbon content (C%), and energy recovery factor of the hydrochars according to the biomass composition and the process severity. The results revealed that the interactions between the biomass components do not have a major effect on the hydrochar characteristics, whereas the interaction between cellulose and SF is the most significant. Moreover, it was shown that, after the lignin content, the hemicellulose content has the highest positive impact on HHV and C% of the hydrochar. An optimization study showed that, with a focus on minimizing the SF while the HHV is maximized, biomass with a cellulose content of 40%, hemicellulose of 35%, and lignin of 25%, under the severity of 4.41 will be the most suitable case for HTC treatment. Finally, a comparison between the predictions and the experimental data in the literature suggests that the proposed equations can provide a good evaluation on the HTC of several biomass feedstocks, especially when the amount of ash and extractives are low.