The purpose of this study is to develop a new and simple methodology for calculating mean size-specific dose estimates (SSDE) over the entire scan range (mSSDE) from weight and volume CT dose index (CTDI_vol). We retrospectively analyzed data from a dose index registry. Scan areas were divided into two regions: chest and abdomen–pelvis. The original mSSDE was calculated by a commercially available software. The conversion formulas for mSSDE were estimated from weight and CTDI_vol (SSDE_weight) in each region. SSDE_weight were compared with the original mSSDE using Bland–Altman analysis. Root mean square differences were 1.4 mGy for chest and 1.5 mGy for abdomen–pelvis. Our method using formulae can calculate SSDE_weight using weight and CTDI_vol without a dedicated software, and can be used to calculate DRL to optimize CT exposure doses.