CO2 level is increasingly used for the determination of the occupancy of rooms. In the present work, miniature CO2 sensors were installed within the Lilliad Learning Center of the University of Lille, a 3-level building receiving daily up to about 5000 patrons. The measured concentrations can be related to the daily number of occupants inside the building, with reasonable precision. This demonstrates that the use of CO2 sensors for occupancy determination is therefore not restricted to single rooms, but is also feasible in a whole building, with no assumption on its architectural or ventilation parameters.