A computational method for calculating the geometric factors of an instrument detecting charged particles in the energy range of about 5-500 keV is presented. The method takes into account the presence of electric or magnetic fields that are intentionally generated to clearly separate electrons from positive ions. The method first solves the distribution of electric or magnetic fields near the detectors, and then calculates the trajectory of scattered and unscattered charged particles under the influence of the calculated fields. The propagation of charged particles through the fields, their interaction with the instruments, and energy deposition into the detectors are calculated with Geant4, whereas the electric or magnetic fields are solved with SIMION. To geometrically model the shielding distribution of the instrument, a novel method is introduced for interfacing with the sophisticated mechanical designs available from computer-aided design tools. A description of this computational method is provided, along with the results for a representative example. The calculation applied to the example clearly demonstrates the necessity of proper accounting of interaction mechanisms such as scattering or secondary emission. This procedure will demonstrate a precise method for calculating the geometric factor that allows estimation of the fluxes of incident charged particles. (C) 2013 Elsevier B. V. All rights reserved.