Objective The authors studied the factors associated with postoperative drainage volume after burr-hole craniostomy for chronic subdural hematoma (CSDH) to predict real postoperative drainage volume, and the relation between brain expansion and drained hematoma volume. Methods During the last 7 years, 75 patients with CSDH had been undertaken burrhole drainage. For these, demographical and radiological data regarding CSDH were gathered and reviewed, including hematoma density, volume, and parameters of brain compliance on computed tomography (CT) scan. We estimated the brain expansion using the degree of frontal horn compression and midline shifting by the B/A ratios, and made statistical analyses for relation with the postoperative drainage volume. Results In a whole, the estimated volume was 139 mL, mean real drainage volume was 261 mL, and mean B/A ratio was 0.53. There was no statistical significance between estimated and real drainage volumes. Among 4 groups, low density subgroup showed the largest difference between estimated volume (128 mL) and mean real drainage volume (214 mL), and lowest mean B/A ratio of 0.5. B/A ratio had inverse relationship with real drainage volume (p<0.05). Conclusion The postoperative drainage volume after operation of CSDH was affected by brain expansion. Postoperative drainage volume was decreased as the B/A ratio was increased, and vice versa. Postoperative drainage volume of CSDH, in some extent, can be predicted with the density types of CSDH and the degree of frontal horn compression.