Aqueous pKa values are essential for understanding the behaviour of chemical species, such as those used as drugs, in buffer systems, or to study enzyme performance and antioxidant activity. Phenolic compounds play important roles in these applications; however, there are limited experimental aqueous pKa values reported. In this study, three calculation schemes, five functionals (M06-2X, B3LYP, BHandHLYP, PBE0, TPSSTPSS), and two solvent models (SMD and IEFPCM) are used to predict pKa values using a set of 15 phenols to evaluate the accuracy of these predictions. Nitrophenols were not predicted well in any scheme, hence mean absolute errors (MAE) were calculated including and excluding these molecules. The best performance (with a MAE of 0.46 pKa units excluding nitrophenols) was achieved at the M06-2X(SMD)/6-311++G(d,p) level of theory using the acid-base equilibrium: HA + Ref- ⇌ A- + HRef, where HRef is the reference acid used (phenol). Two other functionals, B3LYP(SMD) and BHandHLYP (SMD), were quite similar in performance with MAE of 0.57 and 0.55 pKa units, respectively (excluding nitrophenols). Aqueous pKa predictions were performed for a set of 13 phenolic antioxidants, and 10 pharmaceutically interesting phenolic cannabinoids. These 23 molecules are the subject of kinetic studies by our group for which their aqueous pKa values are needed.