BACKGREOUND: Several formulas for estimating allowable pre-transfusion blood loss were used to reduce unnecessary intraoperative blood replacement. The postoperative predicted hematocrit computed by formula was compared with the actual hematocrit and was tested which formula was more accurate in spine surgery. METHOD: Total blood volume was estimated in spine surgery of 34 patients. The target hematocrit (Hct) was suggested on 30% and the allowable blood loss was computed using the formula 1 and 2. For each patients, simultaneous measurement of blood loss and Hct was obtained at the end of operation. The postoperative predicted Hct by the formula 1 and 2 was calculated and compared with the actual Hct, and the difference between the formula 1 and 2 was evaluated. Allowable blood loss=Estimated blood volume (Initial Hct Target Hct)/Initial Hct ... Formula (1) Allowable blood loss=Estimated blood volume (Initial Hct Target Hct)/Average Hct ... Formula (2) RESULTS: 1) The preoperative Hct was 40.7 3.9%. The postoperative predicted Hct by the formula 1 and 2 were 34.3 4.6 and 34.9 4.3% respectively. The postoperative actual Hct was 30.1 4.6%.2) The difference between the predicted Hct by the formula 1 and the actual Hct was 4.2% (P<0.05). The difference between the predicted Hct by the formula 2 and the actual Hct was 4.8% (P<0.05). The difference between the predicted Hct by the formula 1 and 2 was 0.6% (P<0.05). It was thought that the predicted Hct by the formula 1 was more closer to the actual Hct. CONCLUSIONS: The predicted Hct by both formulas is underestimated when the results compare with the actual Hct. But the predicted Hct by the formula 1 provides a closer results to the actual Hct than the predicted Hct by the formula 2.