Abstract Many buildings in warm humid climates, particularly in tropical regions, rely for much of the time on natural ventilation from prevailing breezes for indoor thermal comfort. Much effort in recent years has been directed toward the use of computational fluid dynamics in evaluating airflow through buildings based on the solution of Navier-Stokes equations incorporating a turbulence model. This approach requires extensive data preparation and a reasonably powerful computer to yield results within an acceptable computation time for both numerical solution and simulated flow visualisation. Quantitative evaluation of natural ventilation through many low budget buildings in tropical regions is not evaluated due to a lack of suitable simple computer programs. What is needed are programs that can run on modest personal computers and be used quickly to compare the relative natural ventilation performance of alternative building layouts for prevailing breeze directions during the preliminary design stage. Smaller buildings are often designed for cross ventilation by prevailing breezes with flow entering a windward opening and exhausting through a leeward opening. Such flow through a limited number of openings in series can be calculated very quickly on a personal computer using an orifice flow approach based on estimates of pressure differences and discharge coefficients of openings. When buildings have external ventilating openings in a number of rooms and flow branches within the building, it is no longer possible to calculate directly the airflow in the various branches of the airflow network. Flow in such networks can be analysed iteratively on a personal computer by repetitive solution of simultaneous equations for flow rates in branches at nodes and conservation of mass flow through the network. The procedure described in the paper uses the Hardy Cross method of balancing flows at network nodes until errors throughout the network are acceptably small. Sources of data on wind pressure distributions over building walls and shielding influence of nearby buildings are provided together with a detailed description of a procedure for solving network airflows sufficient for readers to write their own computer code.