CFM Calculator

Find the airflow a room or system needs, from air changes per hour, heat load, or air conditioner size.

Calculate CFM From
Fractions and decimals both work.
–
CFM
Room volume
Air changes
Metric
Liters per second
Per Hour

Airflow List

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How to Calculate CFM

CFM is cubic feet of air per minute. Three common ways to find how much a room or system needs:

Air changes: CFM = room volume (ft³) × air changes per hour ÷ 60
Heat load: CFM = sensible BTU/hr ÷ (1.08 × temperature difference °F)
Air conditioning: CFM = tons × 350 to 450 (400 typical)

1.08 is the density of standard air (0.075 lb/ft³) × its specific heat (0.24 BTU/lb·°F) × 60 minutes.

Worked Example

Tips

Required air change rates depend on the room's use and your local mechanical code; ASHRAE 62.1 and 62.2 set ventilation for commercial and residential buildings, and bathroom and kitchen exhaust fans have code minimums. For duct sizing from CFM, see the duct size calculator.