CFM Calculator
Check our CFM calculator to find the required airflow in cubic feet per minute (CFM) for any room size, depending on its recommended air change per hour.
Airflow needed320 CFM543.7 m³/h
Room volume3,200 cu ft90.61 m³
Air changes per hour6
Airflow320 CFM151.01 l/s
Minutes per air change10
Duct size at 700 ft/min9.16 in rounda common design velocity for residential branches
Duct size at 900 ft/min8.07 in roundtrunk ducts run faster
Comparable toa bedroom or living roomamong 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.