The CT scanner-displayed radiation dose information is based on CT dose index (CTDI) over an integration length of 100 mm (CTDI(100)), which is lower than the CTDI over an infinite integration length (CTDI(∞)). In an adult or a pediatric body CT scan, the limiting equilibrium dose can be established near the central scan plane, and CTDI(∞) more closely indicates the accumulated dose than CTDI(100). The aim of this study was to (a) evaluate CTDI efficiencies, ϵ(CTDI(100)) = CTDI(100)/CTDI(∞), for a multi-detector CT (MDCT) scanner, (b) examine the dependences of ϵ(CTDI(100)) on kV, beam width, phantom diameter, phantom length and position in phantom and (c) investigate how to estimate CTDI(∞) based on the CT scanner-displayed information. We performed a comprehensive Geant4-based simulation study of a clinical CT scanner, and calculated ϵ(CTDI(100)) for a range of parameters. The results were compared with the ϵ(CTDI(100)) data of previous studies. Differences in the ϵ(CTDI(100)) values of these studies were assessed. A broad analysis of the ϵ(CTDI(100)) variations with the above-mentioned parameters was presented. Based on the results, we proposed a practical approach to obtain the weighted CTDI(∞) using the CT scanner-displayed information. A reference combination of 120 kV and a beam width close to 20 mm can be selected to determine the efficiencies of the weighted CTDI by using either phantom measurements or computer simulations. The results can be applied to estimate the weighted CTDI(∞) for 80-140 kV and the beam widths within 40 mm. Errors in the weighted CTDI(∞) due to the variations of kV and beam width can be 5% or less for the MDCT scanners.