Air Changes per Hour, or ACH, is the amount of times the volume of air in a room will be replaced in one hour. ACH = (60 x CFM)/V 1. CFM represents airflow in cubic feet per minute (CFM or ft3/min). Use only supply air CFMs or return air CFMs, whichever is larger, to calculate ACH. Do not use both!
2. V is the volume of the room in cubic feet (ft3). 3. 60 is a constant for the number of minutes in an hour.