Whole House Fan Size Calculator
Wondering what size whole house fan do I need? The answer comes from your home's air volume: a whole house fan should be powerful enough to replace all the indoor air every 2–3 minutes. Enter your floor area and ceiling height below and this calculator gives you the recommended CFM range, the air changes per hour it produces, and how much attic vent area that fan will need.
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How to use this calculator
Enter the total floor area of the living space the fan will cool, in square feet — for example, a single-story 1,800 sq ft home, or the square footage of the cooled floors in a two-story home. Then enter your average ceiling height in feet. The calculator recalculates instantly as you type.
Your main answer is a CFM range. The low end exchanges all indoor air about every 3 minutes; the high end does it every 2 minutes. If your climate has warm but not extreme evenings, the middle of the range is a sensible target. Rounding up slightly is fine — you can always run a bigger fan on a lower speed, but a fan that is too small cannot catch up on a hot evening.
Do not skip the venting number. A whole house fan pushes house air up into the attic, and that air must escape through attic vents. If the attic is under-vented, the fan pressurizes the attic and airflow drops. The table below shows the vent area needed for both ends of your CFM range.
How it works
First the calculator finds your home's air volume: floor area × ceiling height (cubic feet). For the default inputs that is 1,800 × 8 = 14,400 cu ft. The industry rule of thumb is to size the fan so it moves the entire volume of indoor air every 2–3 minutes.
Recommended CFM therefore runs from volume ÷ 3 (a full air exchange every 3 minutes) to volume ÷ 2 (every 2 minutes). With the defaults: 14,400 ÷ 3 = 4,800 CFM up to 14,400 ÷ 2 = 7,200 CFM. Air changes per hour (ACH) is simply CFM × 60 ÷ volume, which gives 20 ACH at the low end and 30 ACH at the high end.
Attic venting uses another rule of thumb: about 1 sq ft of net free vent area per 750 CFM of fan capacity. So the 4,800 CFM end of the range needs roughly 4,800 ÷ 750 = 6.4 sq ft of vent opening, and the 7,200 CFM end needs 7,200 ÷ 750 = 9.6 sq ft. Ridge vents, soffit vents, and gable vents all count toward this total; screens and louvers reduce a vent's net free area, so published ratings matter.
These are planning estimates based on the most common rules of thumb, not a substitute for a manufacturer's sizing guide or an HVAC professional's assessment — ceiling height variations, open floor plans, and local climate all affect the ideal choice.
A final honest note on what these fans actually do: a whole house fan cools by replacing hot indoor air with cooler outdoor air. It shines in the evening, when outside temperatures drop below indoor ones. It is not air conditioning — in hot, humid weather when the air outside is as warm as the air inside, it cannot cool the house, and running it in humid conditions can pull moisture into the home. Many homeowners use a whole house fan to delay or reduce AC use, not eliminate it.
Frequently asked questions
What is the difference between a whole house fan and an attic fan?
They do different jobs. A whole house fan sits in the ceiling between the top floor and the attic: it pulls hot air out of your living spaces and pushes it into the attic, drawing cooler outdoor air in through open windows. An attic fan (or attic ventilator) only ventilates the attic itself and does not cool the rooms below. A whole house fan needs an attic with enough exhaust vents, because the air it pushes up has to escape somewhere.
How much attic venting do I need for a whole house fan?
A common rule of thumb is 1 square foot of net free vent area for every 750 CFM of fan capacity. A 7,200 CFM fan therefore needs about 9.6 square feet of vent opening, typically provided by a combination of ridge vents, soffit vents, and gable vents. If the attic is under-vented, the fan pressurizes the attic and performance drops, so the calculator above shows the vent area needed for both ends of your CFM range.
When should I run my whole house fan?
Run it when the outdoor air is cooler than the indoor air, most often in the evenings and at night. Open a few windows (at least two or three) so fresh air can flow through the house and up into the attic. Turn it off and close the windows during the heat of the day, and always follow the manufacturer's guidance on whether to run it in humid conditions.
Are whole house fans noisy?
They can be. Older direct-drive whole house fans are noticeably loud, while newer models with insulated ducts, remote motors, and vibration isolation are much quieter. Look for the sones rating when shopping. Also look for an insulated, airtight damper or cover: it seals the fan opening in winter so heated air does not escape through the ceiling fan's louvers.