The Cockcroft–Gault (C–G) equation is widely used for drug dose settings in Japan. However, several reports have questioned its accuracy. In previous decades, estimation equations of creatinine clearance (CCr), such as the Orita–Horio equation, have been established based on the Japanese population. We previously built the fitted C–G and fitted Orita–Horio equations by fitting the coefficients of the estimation equations to the study population. However, the usefulness of these equations for drug dose settings remains unclear. Our preliminary study verifies the accuracy of these equations by comparing the predictive performance of the initial vancomycin (VCM) trough value between four equations: the conventional C–G (as control), conventional Orita– Horio, fitted C–G, and fitted Orita–Horio equations. Patients receiving VCM intravenously between January 2015 and March 2019 at Hokkaido University Hospital were enrolled. Overall, 308 patients were included. As initial dose setting methods, we selected two therapeutic drug monitoring (TDM) analysis software: SHIONOGI-VCM-TDM ver.2009 (VCM-TDM) and Vancomycin MEEK TDM analysis software Ver2.0 (MEEK). Predictive performances were evaluated by calculating mean prediction error and mean absolute prediction error (MAE). The lowest MAE was obtained with the conventional C–G equation using VCM-TDM, indicating high predictive performance. However, contrasting result was obtained with MEEK, where the highest MAE was obtained using conventional C–G equation. Moreover, no significant differences were observed in MAE between the other three equations, suggesting that accurate dose settings are not always achieved, despite using accurate CCr equations based on the Japanese population.