To determine air changes per hour, it is necessary to know the volume of the room or contained space and the actual output of the filter, air purifier or other ventilation equipment supplying air to the space. The number of air changes per hour is the number of times one volume of air is replaced by a fresh supply of air in the space per hour.
To calculate the air change rate per hour, the velocity of the fresh air supplied by the ventilation equipment is multiplied by 60, with the velocity expressed in terms of cubic feet per minute. This number is then divided by the volume of the room in cubic feet. The volume of the room is calculated by multiplying the room's length by its width and height. The formula is n = 60q / V.
Building code regulations, particularly mechanical specifications and ventilation design standards, usually specify the minimum required air change rate for different classes of buildings and certain types of rooms. For example, the California Mechanical Code specifies the minimum air changes per hour required in hospital cleanrooms to contain airborne infection. In another example, U.S. Occupational Safety and Hazard Agency standards require a flammable storage room have six air changes per hour.