Whole House Fan CFM Calculator
Fan size and attic vent area for whole-house cooling.
Enter your values
ⓘ Live results
Results
- Fan size
- 4,800 CFM
- House volume
- 14,400 cu ft
- Attic exhaust free area
- 6.4 sq ft (922 sq in)
- Window opening area
- 12 sq ft total
- CFM per sq ft
- 2.67
Quick answer
Whole house fans are sized for 15–23 air changes per hour, roughly 2–3 CFM per square foot of living space.
CFM = floor area x height x ACH / 60
How to use the Whole House Fan CFM Calculator
- 1Enter the conditioned living area and average ceiling height.
- 2Pick 15 ACH for mild climates or 20–23 for hot ones.
- 3Open enough windows and provide the attic exhaust area shown before running the fan.
Worked example
Using the values the calculator loads with:
Inputs
- Living area1800 sq ft
- Average ceiling height8 ft
- Target air changes20 ACH
Results
- Fan size4,800 CFM
- House volume14,400 cu ft
- Attic exhaust free area6.4 sq ft (922 sq in)
- Window opening area12 sq ft total
- CFM per sq ft2.67
Frequently asked questions
What happens with too little attic venting?
Pressure builds, airflow drops and the fan gets loud — you need roughly 1 sq ft of net free attic vent per 750 CFM.
When should I run it?
When outdoor air is cooler than indoor, usually evening through early morning.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.