Background: In the tube current modulation (TCM) technique, tube current is changed dynamically during the scanning process. To quantify the effect of a dynamic tube current, a distinct calculator is needed to estimate the CT output radiation dose in terms of volume CT dose index (CTDI_vol) and individual patient dose in terms of size-specific dose estimate (SSDE). This study developed a specific calculator for CT scanning using the TCM technique. Materials and Methods: The tube current was extracted from the DICOM header for every slice, and averaged over the scan length. The water equivalent diameter (D_w) and SSDE values were calculated for each tube rotation. The software was retrospectively applied to 57 patients who had undergone abdominal and thoracic CT examinations using a multi-detector CT, the Somatom Emotion 6.  Results: The differences between the calculated CTDI_vol and the CTDI_vol reported by the CT scanner were 4.4 ± 1.2% and 6.0 ± 2.0% for abdominal and thoracic examinations, respectively. The average tube current was found to be linearly correlated with D_w with R² values of 0.707 and 0.696 for abdominal and thoracic examinations, respectively. The average tube current was also linearly and strongly correlated with the SSDE with R² values of 0.941 and 0.887 for abdominal and thoracic examinations, respectively. Conclusion: Calculator for estimating CTDI_vol and SSDE specifically for TCM in CT scanning has been successfully developed. The difference between calculated CTDI_vol values using this calculator and reported CTDI_vol values were less than 10%.