Air Changes per Hour Calculator
With the air changes per hour calculator (ACH) you can determine how often your air device exchanges the air in a room.
Air changes per hour7.5 ACH400 CFM through 3,200 cu ft
Room volume3,200 cu ft90.61 m³
Air changes per hour7.5
Airflow400 CFM188.76 l/s
Minutes per air change8
Duct size at 700 ft/min10.24 in rounda common design velocity for residential branches
Duct size at 900 ft/min9.03 in roundtrunk ducts run faster
Comparable toa bathroomamong the usual recommended rates
The formula
CFM = volume × ACH ÷ 60; ACH = CFM × 60 ÷ volume
Volume, not area
Air changes are about the volume of air in the room, so a room with a 12-foot ceiling needs half again as much airflow as the same floor area under 8 feet. The 60 in the formula is just minutes per hour, converting CFM into cubic feet per hour.
Recommended rates vary by use rather than by preference: six changes an hour suits a living space, a bathroom wants eight, a kitchen fifteen, and a space with fumes or dust needs far more. Those are ventilation rates, quite separate from the much larger airflow a heating or cooling system moves.