The CFM formula
CFM = room volume (ft³) × air changes per hour ÷ 60. A 15×12 room with 8-ft ceilings holds 1,440 ft³; at 6 ACH it needs 144 CFM of airflow. For HVAC supply planning, the quick cross-check is ~1 CFM per square foot of floor area, and 400 CFM per ton of cooling system-wide.
Recommended air changes per hour
| Space | ACH | Notes |
|---|---|---|
| Bedrooms | 3–5 | comfort ventilation |
| Living areas / offices | 5–7 | standard comfort |
| Kitchens | 7–9 | plus range hood (100–400 CFM ducted) |
| Bathrooms | 8+ | code minimum exhaust usually 50 CFM |
| Workshops/gyms | 8–12 | fumes, humidity, odor |
Exhaust fans: the two sizing shortcuts
Bathrooms: 1 CFM per square foot, minimum 50 CFM (add 50 for a jetted tub or separate WC). Kitchens: range hoods are sized to the cooktop — 100 CFM per linear foot of hood for gas ranges as a floor, more for high-BTU burners. Both must actually duct outside; recirculating fans don't count as ventilation.
Frequently asked questions
How many CFM do I need per square foot?
For general HVAC supply, roughly 1 CFM per square foot of living space. Ventilation-driven rooms are sized by air changes instead: volume × ACH ÷ 60.
How many CFM for a bathroom fan?
Code minimum is typically 50 CFM; the rule of thumb is 1 CFM per sq ft of bathroom. A 10×8 bath needs ~80 CFM. Long duct runs justify the next size up.
What does ACH mean?
Air changes per hour — how many times the room's entire air volume is replaced in one hour. Bedrooms need 3–5; kitchens and baths 8+.
How many CFM per ton of AC?
The design standard is 400 CFM per ton (350 in humid climates for better dehumidification, up to 450 in dry climates).