This paper presents a guideline for selecting ventilation fan based on empirical data. The research project was undertaken in a university classroom of 64 square meters. In a two-hour experiment, carbon dioxide levels exceeded 1600 ppm when not using any ventilation system. Subsequently, the carbon dioxide levels increased approximately at a rate of 11.49 ppm per minute for 15 students sitting in the room. The carbon dioxide levels within the tested classroom exceeded 1000 ppm after the 42nd minute. When students left the room, CO2 decreased approximately at a rate of 3.67 ppm per minute. Straight-line functions were used to model the increasing and decreasing behaviour of CO2 concentration in the classroom. An example of how to select an appropriate ventilation fan capacity is shown as an approximation guideline.