Introduction: Anticoagulation is essential during CPB in cardiac surgery. It is generally performed using heparin; however the widely used formula for its dosing is based solely on body weight. Although the formula assumes activated clotting time (ACT) to be within normal range, baseline ACT varies in each patient. Thus, we developed an original formula, which takes into account baseline ACT in addition to body weight to calculate a more proper dose for initial administration of heparin. In this study, we monitored the ACT to examine if the dose of heparin calculated using our formula can prolong the ACT to the target range, and we determined the factors which interfere with the prolongation of ACT. Methods: Between October 2010 and April 2011, 141 consecutive patients underwent cardiac surgery requiring cardiopulmonary bypass at our hospital. We measured ACT 3 minutes after the initial administration of heparin and considered ACT values >400 seconds as appropriate for safe initiation of CPB. Results: Using the proposed formula, administered heparin dose was 241 ± 27 IU/kg and target ACT was achieved in 86.4% of patients. Multivariate analysis was performed to determine the effect of patient background factors on target ACT achievement. Body weight, age, and preoperative heparin therapy, which showed significant differences, were further analyzed. Conclusions: This study demonstrated that our newly developed formula could be used to properly calculate the optimal initial dose of heparin.