Whole House Fan Size Calculator: CFM and Attic Venting
Size a whole house fan by the air it moves: square feet × ceiling height × air changes per hour ÷ 60. Most homes target a full air change every 2 to 4 minutes, and California's Energy Code sets a floor of 1.5 CFM per square foot. The attic must let that air out: at least 1 square foot of net free vent area for every 750 CFM. Enter your home to get the fan size and venting.
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Your project
20 = a full air change every 3 minutes. Use 15 for mild nights, 30 for hot, dry climates.
Your results
- Fan size
- 5,333 CFM
- Attic vent net free area
- 7.1 sq ft
- 1 sq ft per 750 CFM. Most vents list their net free area on the label.
- By air changes
- 5,333 CFM
- 16,000 cu ft × 20 ÷ 60
- Title 24 minimum (1.5 CFM/sq ft)
- 3,000 CFM
- One full air change every
- 3 minutes
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
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How this calculator works
- Volume = floor area × ceiling height.
- Airflow by air changes = volume × ACH ÷ 60.
- Fan size = the larger of that and 1.5 CFM × floor area (California Title 24, Part 6, §150.1(c)12).
- Attic net free vent area = fan CFM ÷ 750 sq ft.
Whole house fan size by home size (8 ft ceilings, 20 air changes per hour)
| Home | By air changes | Title 24 minimum | Fan size | Attic vent area |
|---|---|---|---|---|
| 1,000 sq ft | 2,667 CFM | 1,500 CFM | 2,667 CFM | 3.6 sq ft |
| 1,500 sq ft | 4,000 CFM | 2,250 CFM | 4,000 CFM | 5.3 sq ft |
| 2,000 sq ft | 5,333 CFM | 3,000 CFM | 5,333 CFM | 7.1 sq ft |
| 2,500 sq ft | 6,667 CFM | 3,750 CFM | 6,667 CFM | 8.9 sq ft |
| 3,000 sq ft | 8,000 CFM | 4,500 CFM | 8,000 CFM | 10.7 sq ft |
Attic vent area is net free area (NFA), not the vent's outside dimensions.
How a whole house fan works
A whole house fan sits in the ceiling of a central hallway and pulls cool evening air in through open windows, pushing the warm house air up into the attic and out through the attic vents. It replaces air conditioning on nights when the outside air is cooler than inside, at a fraction of the power. It only works with windows open, and the attic vents have to be big enough to let the air out — undersized vents push hot attic air back into the house and can backdraft gas appliances.
Attic venting
The 750 CFM per square foot rule is for net free area: the open area of a vent after louvers and screens. A typical 14 × 24 inch gable vent has around 1 sq ft of NFA; a 4-ft ridge vent section and soffit vents each list theirs. Existing roof ventilation counts. If the attic is short, add gable or ridge vents before installing the fan.
Insulation and the ceiling opening
The fan's ceiling opening is a big hole in the thermal envelope in winter. Choose a model with insulated, self-closing dampers, or cover the opening with an insulated box for the heating season, and keep attic insulation away from the fan housing as the manufacturer directs.
Keep going
- Airflow for one room or a bath fan: CFM calculator
- Keep the soffit intakes open under the insulation: ventilation baffle calculator
- Sizing ridge, soffit and gable vents for the whole roof: attic ventilation calculator at Smart Roofing Calculator
Frequently asked questions
What size whole house fan do I need?
Multiply your floor area by ceiling height, then by 20 air changes an hour, and divide by 60. A 2,000 sq ft home with 8 ft ceilings is 16,000 cu ft, so 5,333 CFM; California's minimum for the same home is 3,000 CFM (1.5 per sq ft).
How much attic venting does a whole house fan need?
At least 1 square foot of net free vent area per 750 CFM of fan airflow — about 7 sq ft for a 5,000 CFM fan. That is California Title 24's requirement and the common manufacturer rule.
Is a bigger whole house fan better?
Up to a point. More CFM cools the house faster, but it needs more attic venting and more open window area, and it's louder. Many people buy a two-speed fan sized for hot nights and run it on low.
How many windows need to be open?
Enough that the fan pulls air in quietly instead of straining: open windows in the rooms you want cooled, wider in the bedrooms at night, and follow the fan maker's stated minimum open window area for its size.
Can I use a whole house fan with air conditioning?
Not at the same time. Run the fan in the evening and overnight when it's cooler outside, then close the windows and switch to AC as the day heats up.
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Fan size
5,333 CFM