The periodic calibration of measurement devices, such as force platforms (FPs), guarantees and optimizes the quality of the acquired data. In this study, the theoretical validation of a portable system for the in-situ calibration of six-component FPs is presented. A least-squares algorithm, that estimates the FP re-calibration matrix, was designed and tested using a simulation approach. The algorithm is the core of a portable system, described in a separate paper, and represents a refinement of an algorithm previously presented by Cappello et al. [Cappello A, Lenzi D, Chiari L. Periodical in-situ re-calibration of force platforms: a new method for the robust estimation of the calibration matrix. Med Biol Eng Comput 2004;42:350-5]. The new algorithm assumes that the calibration inputs are known, 3-D, time-varying loads, applied to the FP at known coordinates, and not constrained in their direction of application. Simulation results confirmed the a priori identifiability of the re-calibration matrix and some of the algorithm features were optimized in the perspective of an actual building of the system. With the aid of simple sinusoidal loads, applied in at least five different points, we proved that the algorithm can ensure errors less than 0.2N and 0.4 Nm when calculating force and moment components of an applied load.