Understanding your result
The headline is the fan size to buy, with the air changes it gives and the attic vent area it needs in the sub-line. The stats repeat the fan size, the air changes and the attic vent area. The table shows the house volume, the rule you chose and why it is used, the airflow it asks for, the fan size, the air changes and minutes per change, the attic net free area needed, how your attic compares if you entered its vent area, the window area to open, and the four rules side by side.
The attic vent area row decides whether the fan will deliver its rating. If you entered your existing net free area and it falls short, the notes say by how much; adding soffit or gable vents to close the gap is part of the installation, and our sister site Roofings Calculator sizes attic ventilation in detail.
A whole-house fan and a ceiling fan solve different problems: the ceiling fan size calculator moves air within a room so people feel cooler, while a whole-house fan replaces the house’s air with cooler outdoor air. The air changes calculator converts any fan’s CFM for any room, and the AC running cost calculator shows what the hours the fan replaces would cost on the air conditioner.
How we calculate this
The CFM-per-square-foot and air-change rules are the ones published by the US Department of Energy and the whole-house fan manufacturers; at 8 ft ceilings, 2 CFM per square foot equals exactly 15 air changes per hour. The attic vent rule of 1 sq ft of net free area per 750 CFM is the common manufacturer guidance (some use 1 per 700 to 800), and the window opening of about twice the attic vent area keeps the house from going into negative pressure. The figures are collected with the other airflow rules on the ventilation and exhaust minimums page.
Metric mode accepts the floor area in square metres and shows the airflow in cubic metres per hour and the vent area in square metres.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Standard rule | 2 CFM per sq ft | US DOE Energy Saver; manufacturer sizing guides |
| Hot-climate rule | 3 CFM per sq ft | Manufacturer guidance for fast cool-down |
| Air-change rules | 15 to 20 per hour | Manufacturer and utility guidance |
| Attic vent area | 1 sq ft net free area per 750 CFM | Common whole-house fan manufacturer guidance |
| Window opening | About 2 × the attic vent area | Manufacturer installation guidance |
| Fan sizes | 1,500 to 7,000 CFM | Residential whole-house fans sold in the US |
Assumptions last reviewed October 8, 2026.
The calculator does not model the fan’s curve against the attic’s back-pressure, the outdoor temperature and humidity that decide when running the fan helps, or the condition of the attic insulation around the fan opening. It does not size attic exhaust fans, which are a different product. The guide to humidity and comfort explains when outdoor air is a good trade, and the guide to degree days covers how many hours a climate offers cool evenings.
Two worked examples
A 2,000 sq ft house by the standard rule
2,000 sq ft, 8 ft ceilings, 2 CFM per square foot, attic vents not entered.
- Volume: 2,000 × 8 = 16,000 cu ft
- Airflow: 2 × 2,000 = 4,000 CFM → 4,000 CFM fan
- Air changes: 4,000 × 60 ÷ 16,000 = 15 per hour, one every 4 minutes
- Attic net free area: 4,000 ÷ 750 = 5.3 sq ft; windows open about 10.7 sq ft
Five and a third square feet of attic vent is about 770 square inches, which a typical house reaches with continuous soffit vents and a ridge vent. The note asks you to add up the vents’ stamped areas, because many attics have half of that.
A smaller house in a hot, dry climate
1,600 sq ft, 8 ft ceilings, 3 CFM per square foot, existing attic net free area 4 sq ft.
- Volume: 12,800 cu ft
- Airflow: 3 × 1,600 = 4,800 CFM → 5,000 CFM fan
- Air changes: 23.4 per hour, one every 2.6 minutes
- Attic net free area needed: 5,000 ÷ 750 = 6.7 sq ft; the attic has 4, short by 2.7 sq ft
- Windows: about 13.3 sq ft open
The hot-climate rule gives a fast evening flush, but the attic is the bottleneck: with 4 sq ft of vents the fan will deliver well under its rating and push hot attic air back through the ceiling. Adding about 390 square inches of net free vent area, for example a few gable vents or more soffit vents, is part of the job.
Where to find your inputs
Floor area. The floors the fan will draw air through, usually the whole house; a basement with closed windows does not count.
Ceiling height. The average across those floors.
Attic net free vent area. Stamped on each soffit, ridge, gable and roof vent as “NFA” in square inches; add them up and divide by 144 for square feet.
Rule. 2 CFM per sq ft for most climates; 3 for hot, dry climates where evenings are cool.
Common mistakes
- Running the fan with windows closed. It depressurises the house and can backdraft gas appliances.
- Too little attic venting. The fan loses airflow and pushes hot air back down.
- Running it while the air conditioner runs. It pulls the cooled air out of the house.
- Running it when outside is hotter or very humid. It only helps when outdoor air is cooler and drier.
- An uninsulated damper. Choose an insulated, sealed damper, or the opening leaks heat all winter.
- Confusing it with an attic fan. Attic fans exhaust the attic only and are sized differently.
Questions people ask
- What size whole-house fan do I need?
- The standard rule is 2 CFM per square foot of floor area, which for a 2,000 sq ft house means a 4,000 CFM fan and about 15 air changes per hour with 8 ft ceilings. In hot climates where the aim is a fast evening cool-down, 3 CFM per square foot is common, so 6,000 CFM for the same house. The air-change rules give similar answers, 15 air changes per hour for 4,000 CFM and 20 for about 5,300.
- How much attic ventilation does a whole-house fan need?
- About 1 square foot of net free vent area for every 750 CFM of fan, according to most fan makers, so 5.3 sq ft for a 4,000 CFM fan. Net free area is the open area of the vents after screens and louvres, stamped on each vent; a typical ridge vent provides about 18 sq in per linear foot and a 4 × 16 inch soffit vent about 25 sq in. Without enough vent area the fan pressurises the attic and loses much of its airflow.
- How many windows should be open when a whole-house fan runs?
- Enough to give about twice the attic vent area in open window area, roughly 10 to 11 sq ft for a 4,000 CFM fan. In practice that is two or three windows open about 6 inches, or more windows open less, spread through the rooms you want to cool. Never run the fan with all windows closed; it will depressurise the house and can pull exhaust gases down the flue of a gas water heater or furnace.
- Is a whole-house fan cheaper than air conditioning?
- When the outdoor air is cooler than indoors, much cheaper. A 4,000 CFM whole-house fan draws about 200 to 600 watts against 2,400 or more for a 3-ton air conditioner, and it can bring a house down 10 to 15 °F in an evening. It does nothing when it is hotter outside than in, and it brings in outdoor humidity, so it works best in dry climates and in the spring and autumn shoulder seasons elsewhere.
- What is the difference between a whole-house fan and an attic fan?
- A whole-house fan sits in the ceiling between the living space and the attic and pulls cool outdoor air in through open windows, through the house, into the attic and out through the attic vents. An attic fan sits in the roof or gable and only exhausts hot air from the attic. Whole-house fans cool the house; attic fans cool the attic, with debated benefit, and are sized differently.
- Are whole-house fans noisy?
- Older belt-drive fans with louvred shutters were loud; modern ducted and insulated-damper models are much quieter because the fan sits in the attic on a flexible duct, away from the ceiling grille. A two-speed or variable-speed fan run on low for most of the evening is quieter still. Insulated, sealed dampers also stop the opening from leaking heat all winter, which was the other complaint about older fans.