Developing predictive models for gas flow through micro- and nanochannels is of great interest in several scientific and technological fields. Existing theories reproduce specific scenarios failing to give solutions that can be applied on a broader spectrum. In this study, we propose a statistical method to predict the flow rate of rarefied gas through rectangular channels based on the distribution of free paths between inter-particle and gas-wall collisions. Our approach can be applied to virtually all geometries, for which the probability distribution of path lengths for gas-wall collisions can be computed, either analytically or by numerical simulations. Additionally, we present a study of nitrogen transport through a wide range of identical slit nanochannels where only the cross section height varies from 250 nm down to 2.5 nm achieving various degrees of gas confinement. The present theoretical model shows excellent agreement with the experimental results demonstrating the validity of our approach.