To calculate the total air exchange rate, you need to determine the total air supplied to and removed from the premises. This can be done by summing up the airflow rates of all supply and exhaust fans, as well as any natural ventilation sources. The total air exchange rate is then divided by the volume of the premises to obtain the air changes per hour (ACH). For example, if the total air supplied is 10,000 CFM and the premises has a volume of 50,000 cubic feet, the total air exchange rate would be 10,000 CFM / 50,000 cu.ft = 0.2 air changes per hour.