Abstract: An apparatus and method indirectly determines the amount of outside air introduced into the ventilation system of a structure. Carbon dioxide concentrations are measured for return air, outside air and mixed air. The flow rate of mixed air is measured. The volume of outside air introduced into the system is determined without directly measuring the volume or flow rate of outside air. For calibrating the apparatus, a return air sensor is calibrated to the outside air and a mixed air sensor is calibrated to the outside air or the return air to eliminate inaccuracy due to sensor drift.