Abstract: Objective To explore a method for calculating water equivalent diameter (Dw) based on localizer CT images for calculation of the size specific dose estimates (SSDE). Method GE Revolution CT and LightSpeed VCT were used to scan CT dose index phantoms 16 cm and 32 cm in diameter at the tube voltages of 80, 100 and 120 kV to obtain the axial image and anteroposterior localizer radiograph. According to the definition of CT Hounsfield unit, the axial images were used to calculate the conversion factors that convert the phantom thickness to water equivalent thickness. The gray value of the localizer radiograph and the water equivalent thickness were calibrated with a linear equation, and the parameters of the calibration were used to calculate the water equivalent thickness. The method was verified using 2 CT dose index phantoms and in 22 patients undergoing chest and abdominal CT examination. Result Comparison of the water equivalent diameter (Dw) based on the localizer radiograph and axial image of the 2 phantoms showed that the percentage difference between Dw from the axial images and from the localizer radiograph was below 3% . The trend of Dw variations with location in the two methods was sonsistent. The difference in Dw in intermediate region of interest between the axial image and the localizer radiograph from the 22 patients was below 6.6% . With the mean Dw in the ROI, the maximum percentage difference was 7.5% . Conclusion Calibration of the gray value of the localizer radiograph and the water equivalent thickness using the axial image and localizer radiograph of CT dose index phantoms allows quick calculation of the SSDE based on the parameters of calibration.