Whole House Fan CFM Calculator
Fan size and attic vent area for whole-house cooling.
Enter your values
Enter the living area in sq ft.
Enter the average ceiling height in ft.
Enter the target air changes in ACH.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Fan size and attic vent area for whole-house cooling. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Living area, Average ceiling height, Target air changes |
| Main output | Fan size |
| Formula | CFM = floor area x height x ACH / 60 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Living area(sq ft)
- Enter the living area in sq ft.
- Average ceiling height(ft)
- Enter the average ceiling height in ft.
- Target air changes(ACH)
- Enter the target air changes in ACH.
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.
What do I need to enter into the Whole House Fan CFM Calculator?
Just 3 values: living area, average ceiling height, target air changes. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Whole House Fan CFM Calculator work out the answer?
Whole house fans are sized for 15–23 air changes per hour, roughly 2–3 CFM per square foot of living space. It applies the formula CFM = floor area x height x ACH / 60 and shows the working so you can check each step by hand.
Is the Whole House Fan CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.