This study reports on the computational analysis and experimental calibration of the whole-body counting detection equipment at the Nuclear and Technological Institute (ITN) in Portugal. Two state-of-the-art Monte Carlo simulation programmes were used for this purpose: PENELOPE and MCNPX. This computational work was undertaken as part of a new set of experimental calibrations, which improved the quality standards of this study's WBC system. In these calibrations, a BOMAB phantom, one of the industry standards phantoms for WBC calibrations in internal dosimetry applications, was used. Both the BOMAB phantom and the detection system were accurately implemented in the Monte Carlo codes. The whole-body counter at ITN possesses a moving detector system, which poses a challenge for Monte Carlo simulations, as most codes only accept static configurations. The continuous detector movement was approximately described in the simulations by averaging several discrete positions of the detector throughout the movement. The computational efficiency values obtained with the two Monte Carlos codes have deviations of less than 3.2 %, and the obtained deviations between experimental and computational efficiencies are less than 5 %. This work contributes to demonstrate the great effectiveness of using computational tools for understanding the calibration of radiation detection systems used for in vivo monitoring.