This paper describes the development of a simplified tool which should be used at the early design stage for predicting air ventilation rates in a building. The method is based on the assumption that the air flow rate may be calculated as a function of two independent parameters. The first, called effective pressure difference, takes into account the local weather data, surrounding terrain, and building typology. The second is the overall building permeability, and accounts for permeable components (geometry and permeability), including vents. The model has been built using a multi-zone code (COMIS) as a reference. Four building typologies (single family house, row house, terrace house and high rise building) have been analysed. The effective pressure difference is assumed to be a linear two-parameter function of indoor outdoor temperature difference, and wind speed. The two parameters depend on building typology and plan area density of surrounding terrain. Stability and uncertainty of the model have been checked for different building orientations, envelope permeability, and weather data representative of the heating season. Once the typology, location, and surrounding terrain of the building are known, the designer may find the fraction of time for which natural ventilation is sufficient to provide the desired air flow rate, given the overall permeability. Or, vice versa, he may find the optimal permeability given the fraction of time. This information will be a useful tool to orient the designer in choosing between natural, mechanical or hybrid ventilation systems.