The Horvath-Kawazoe (HK) model that allows computing the filling pressure of micropores is traditionally used in gas porosimeters to assess the pore size distribution of microporous materials because of its simplicity, even if it is known to overestimate the pore sizes. In this study, we propose a new thermodynamic model that is able to link the filling pressure of a slit micropore to its pore size and that describes both primary and secondary filling, which is not feasible with the original HK model. The principal novelty of the pore filling model presented in this work, in comparison to its HK predecessors, is the inclusion of both pore width and pressure in the estimation of the adsorbate density in the pore. Moreover, the model is based on a simplified schematization of the adsorbed fluid that is considered as a combination of dense homogeneous fluid layers parallel to the Ore walls and interacting between them and with the solid. Grand Canonical Monte Carlo simulations have been used to obtain the information concerning the number and density of fluid layers as a function of both pore width and pressure. Even if the method we propose is more general, this thermodynamic model is used in this work to compute, as a first example, the filling pressure of argon and nitrogen at 77K in graphitic slit-like micropores and an excellent agreement is found with non local density functional theory results. Hence, the model is applied to compute, as a first test case, the pore size distribution of a carbon molecular sieve. (C) 2015 Elsevier B.V. All rights reserved.