A whole house fan pulls cool outdoor air in through open windows and pushes the warm indoor air out through the attic. To size one, the Home Ventilating Institute (HVI) multiplies the gross floor area of the house by 3 CFM per square foot, which gives roughly one complete air change every two minutes and a breeze you can feel. A smaller fan at 0.4 CFM per square foot can still cool the structure overnight if ceiling fans provide the breeze. The attic then needs at least 1 sq ft of exhaust opening for every 750 CFM. The calculator and chart below do the arithmetic.
Whole house fan size calculator
Whole house fan CFM chart
| House area | Breeze: fan CFM (x 3) | Attic exhaust area needed | Overnight cooling: fan CFM (x 0.4) | Attic exhaust area needed |
|---|---|---|---|---|
| 1,000 sq ft | 3,000 | 4.0 sq ft (576 sq in) | 400 | 0.5 sq ft (77 sq in) |
| 1,200 sq ft | 3,600 | 4.8 sq ft (691 sq in) | 480 | 0.6 sq ft (92 sq in) |
| 1,500 sq ft | 4,500 | 6.0 sq ft (864 sq in) | 600 | 0.8 sq ft (115 sq in) |
| 1,800 sq ft | 5,400 | 7.2 sq ft (1,037 sq in) | 720 | 1.0 sq ft (138 sq in) |
| 2,000 sq ft | 6,000 | 8.0 sq ft (1,152 sq in) | 800 | 1.1 sq ft (154 sq in) |
| 2,400 sq ft | 7,200 | 9.6 sq ft (1,382 sq in) | 960 | 1.3 sq ft (184 sq in) |
| 2,500 sq ft | 7,500 | 10.0 sq ft (1,440 sq in) | 1,000 | 1.3 sq ft (192 sq in) |
| 3,000 sq ft | 9,000 | 12.0 sq ft (1,728 sq in) | 1,200 | 1.6 sq ft (230 sq in) |
| 3,500 sq ft | 10,500 | 14.0 sq ft (2,016 sq in) | 1,400 | 1.9 sq ft (269 sq in) |
Where does 3 come from? One air change every two minutes in a house with 8 ft ceilings would take 8 / 2 = 4 CFM for every square foot of floor. HVI uses 3 because the gross floor area includes closets, stairs and other spaces that do not need the same airflow; the formula assumes an eight-foot ceiling. If most of your rooms have 9 or 10 ft ceilings, scale the figure up in proportion (about 3.4 or 3.75 CFM per sq ft) to keep the same air change rate.
Use the gross floor area: every floor that the fan will ventilate, including the upstairs of a two-storey house, and the closets. A fan sized from the downstairs alone will feel weak.
Breeze or overnight cooling?
The two rates are for two different jobs. At 3 CFM per sq ft the air moves fast enough through the rooms to feel like a breeze, so it cools people directly in the evening as soon as the outdoor air is cooler than the indoor air. At 0.4 CFM per sq ft the air moves much more slowly; the fan runs through the night and flushes the heat stored in the walls, floors and furniture, so the house starts the next day cooler. HVI notes that the smaller fan relies on other fans, such as ceiling paddle fans, to create the breeze people feel. The smaller fan is cheaper, quieter and needs much less attic vent area, which can be the deciding factor in an older house.
Attic vent area: the part people forget
All the air the fan pushes into the attic has to get out again, and HVI says a whole house fan needs adequate, unobstructed outlets through soffit vents, grilles or louvers: at least 1 sq ft for every 750 CFM. A 6,000 CFM fan needs 8 sq ft of opening. Compare that with the passive vents your attic already has: the attic ventilation calculator works out the net free area the code requires for ordinary attic ventilation, and vent makers print the net free area of each vent on the label. If the existing vents fall short, the fan will be noisy and move less air, and it can pressurise the attic and push attic air back into the house through gaps.
Safety and running it well
- Open windows first. HVI warns that large fans can put the house under significant negative pressure, and at least one window must be open before the fan runs. In practice open several, spread around the house, sized together for more than the attic vents.
- Fuel-burning appliances. A fan pulling air out of a house with too few windows open can draw combustion gases back down the flue of a gas water heater, furnace or fireplace. Never run it with the windows closed.
- Only when it is cooler outside. A whole house fan cools only when outdoor air is cooler than indoor air, so it suits dry climates with cool evenings and nights. In humid weather it brings in moist air.
- Seal it in winter. The ceiling opening is a large hole in the insulated ceiling. Use a fan with an insulated, gasketed damper or fit a cover for the heating season.
- Not an attic fan. A whole house fan ventilates the living space through the attic. A powered attic ventilator only moves air through the attic; see the attic fan size calculator.
Worked example: 1,800 sq ft two-storey house
Ground floor 1,000 sq ft plus upstairs 800 sq ft gives 1,800 sq ft gross. For a breeze: 1,800 x 3 = 5,400 CFM, needing 5,400 / 750 = 7.2 sq ft (1,037 sq in) of attic exhaust opening. For overnight cooling with ceiling fans: 1,800 x 0.4 = 720 CFM and about 1 sq ft (138 sq in) of attic opening. Fans are sold in steps, so pick the nearest size at or above the figure, and check the certified airflow rather than the name on the box.
Building codes also regulate exhaust fans, duct termination and makeup air, and they vary by state and city. HVI’s figures are industry guidance; your building department decides what is required on your job.
Related calculators
- Attic fan size calculator: powered attic ventilators and soffit intake
- Attic ventilation calculator (passive vents, net free area)
- Insulation R-values by climate zone: attic insulation for your county
Frequently asked questions
What size whole house fan do I need?
Multiply the gross floor area of the house, all floors including closets, by 3 for the fan CFM (HVI). A 2,000 sq ft house needs about 6,000 CFM, or about 800 CFM for a smaller fan used to cool the house overnight.
How much attic venting does a whole house fan need?
At least 1 sq ft of unobstructed exhaust opening for every 750 CFM of fan capacity (HVI). A 6,000 CFM fan needs 8 sq ft.
Can a whole house fan be too big?
A fan too large for the attic vents or the open windows moves less air than rated, gets noisy and can depressurise the house. Size it to the floor area and make sure the vents and windows can keep up.
Is a whole house fan the same as an attic fan?
No. A whole house fan draws air through the living space and out through the attic; an attic fan only ventilates the attic.
How many windows should be open with a whole house fan?
HVI says at least one window must be open before the fan runs; in practice open several around the house so the air sweeps every room.
Sources
- Home Ventilating Institute (HVI), How Much Ventilation Do I Need? (HVI ventilation guide articles; read 25 October 2026). Whole-house comfort ventilator capacity (x 3, or x 0.4) and attic exhaust area (CFM / 750).
- HVI, Continuous Whole-House Ventilation (whole-house comfort ventilators, attic outlets, negative pressure warning).
Last reviewed: 25 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.