[en] Estimation of inhalation dose in monazite deposit areas essentially requires concentrations of both radon (²²²Rn) and thoron (²²⁰Rn) for computation. Solid state nuclear track detector (SSNTD) based dosimeters are often employed for the simultaneous measurements of ²²²Rn and ²²⁰Rn. These dosimeters have two identical cups with cellophane in one cup and glass fibre filter in the other for radon alone and radon+thoron entries into the cups, respectively [Eappen, K.P., Mayya, Y.S., 2004. Calibration factors for LR-115 (type-II) based radon thoron discriminating dosimeter. Radiat. Meas. 38, 5-17]. Alpha tracks are registered on SSNTD films inside the cups in proportion to the gas concentrations and their decay progenies. Calibration factors (CFs) are obtained experimentally for known exposure to radon or thoron concentrations that is measured outside the cups. The theoretical CFs for the respective cups are obtained by using track development models for the exposure and etching conditions. In the case of ²²²Rn, it is assumed that the gas inside the cup is uniformly distributed; hence, the CF can be computed with the concentration measured outside the cup. However, in the case of ²²⁰Rn, certain assumptions are required in computing the CF from the gas concentration measured outside the cup due to its short half-life and hence small diffusive length. This paper discusses the theoretical method that is used in calculating the effective concentration of ²²⁰Rn inside the dosimeter cup which is taken for computing the theoretical CF. The CF obtained through modelling is compared with experimental values. Both the values are in very good agreement