The purpose of this study was the comparison of two methods used to calculate the Size Specific Dose Estimate (SSDE) from CT images. The ratio of the SSDE for the two methods for different scan regions was evaluated. The first method used the patient size to find the effective diameter (Deff) and the SSDE while the second method used the patient attenuation water-equivalent diameter (Dw). In this study, the SSDE was calculated using both methods for a retrospective analysis of 10 abdominal, 15 thoracic, and 6 head CT examinations of human adults. The CT scans were segmented automatically to find the body contours. Subsequently, the segmentation results were used to calculate the effective diameter and the effective water equivalent diameter to find the CTDI to SSDE conversion factors for every slice according to the AAPM reports 204 and 220. The total SSDE for a scan is calculated as the average value of all slices. The results of this study show that the ratio Deff/Dw is about 0.98 ±0.06 for abdominal scans, 0.92±0.05 for thorax scans and 1.23 ±0.35 for head scans. From these results, we can conclude that the ratios of Deff and Dw are reasonably constant for chest and abdomen examinations and could be used to determine Dw from Deff. However, due to larger variations of the amount of bone in head CT, in these examinations the difference between Deff and Dw can be more than 50%. Therefore, we recommend to use the water equivalent diameter Dw in reporting the SSDE.