This paper proposes a method for predicting the cross section (XS) of single event upset (SEU) induced by proton direct ionization (PDI). The method is based on the physics-based model previously proposed for both bulk FinFET static random access memories (SRAMs) and planar SRAMs under heavy ion irradiation. The method presented in this paper predicts PDI XS for both 16-nm FinFET and 40-nm planar SRAMs below a few MeV regime, thanks to the broad predictability of our XS model in linear energy transfer (LET) dependence. By calibrating the parameters through heavy ion irradiation, our method enables the prediction of PDI SEU XS in SRAMs, which has not been analytically addressed in previous studies, and its validity has been verified.