A model for prediction of steady-state uniform concentration of a sustained gaseous leak in an enclosure with passive ventilation through one vent is described. Theoretically natural ventilation models under-predict up to twice lower concentrations of releasing gas and over-predict up to twice higher concentrations compared to the model of passive ventilation. The distinctive feature of passive ventilation is positioning of the neutral plane anywhere below the half of the vent height whereas it is located at about half vent height in the case of natural ventilation. The model is compared against experimental data on uniform and non-uniform distribution of helium concentration in the enclosure with one vent of different size and various release flow rates. The model predictions of maximum concentrations of helium observed in the experiments with the conservative discharge coefficient value C D = 0.60 (the best fit range is from 0.60 to 0.95) are closer to measured data than calculation by a model based on the natural ventilation assumptions even with “tuned” C D = 0.25. The engineering nomogram to calculate a release mass flow rate leading to 100% concentration of gas in an enclosure as a function of vent width and height is presented. The equation behind the nomogram is verified by CFD simulations and the appropriate discharge coefficient is derived for use in the equation as C D = 0.85. Effectiveness of different vent configurations is compared based of the ventilation parameter A*H^1/2. A new criterion for mixture uniformity in a ventilated enclosure is suggested and applied to available experimental data. It is concluded that the maximum and minimum mole fractions deviate from the average mole fraction by no more than 20% when the criterion is above 4.