Objectives: A procedure for calculating the absolute blood volume (aBV) before hemodiafiltration (before aBV) of a patient at the beginning of hemodialysis using the continuous relative blood volume measurement and dialysate infusion functions has been reported. In this study, we applied this procedure on patients undergoing intermittent hemodiafiltration (IHDF). Furthermore, aBV after IHDF (after aBV) and excess aBV (overhydrate) were calculated. The association among the parameters used to determine the dry weight (DW) of patients was also examined. Methods: Twenty-eight patients undergoing IHDF using a DCS-200Si versatile dialysis monitoring device (Nikkiso Co. Ltd., Tokyo, Japan) equipped with a BV meter were included in the study. Simultaneously, the plasma values of human atrial natriuretic peptide (hANP) and cardiothoracic ratio (CTR) were also obtained. The csv file from the DCS-200Si, Microsoft Excel, and Visual Basic for application programing were used. Before aBV was measured based on the change in the relative blood volume during five intermittent dialysate infusions of each IHDF. After aBV was measured as a product of before aBV and relative blood volume at the end of the treatment. Overhydrate was computed as the difference between after aBV and 8% of weight after treatment Regression analysis of these parameters was conducted based on the CTR, hANP, and body water composition. Results: After aBV1–after aBV5 and overhydrate1–overhydrate5 significantly correlated with plasma hANP levels. Correlations with overhydrate were higher than those with after aBV (r = 0.490–0.582). After aBV1–after aBV5 were significantly correlated with extracellular water content (r = 0.508–0.545). Conclusions: When the aBV of patients undergoing hemodialysis were calculated using IHDF, we found that after aBV was associated with extracellular water and overhydrate was reflected in cardiac load. Thus, aBV may be a useful indicator of DW.