Whole House Fan Size Calculator

How whole house fan sizing works

A whole house fan pulls cool outside air in through open windows and pushes the hot inside air up into the attic and out through the roof vents. Size is measured in cubic feet per minute, or CFM, and the starting point is the volume of air in the house, which is floor area times ceiling height.

How much of that volume to move per minute depends on how hard you want the air to move. The Department of Energy's consumer guide to home ventilation and the Pacific Northwest National Laboratory's whole house fan guide both put the effective range at about 0.2 to 0.375 CFM per cubic foot of living space, which works out to 12 to 23 air changes an hour. For an 8-foot ceiling that is roughly 1.5 to 3 CFM per square foot of floor area.

This calculator lets you pick the point in that range. The gentle setting is 0.2 CFM per cubic foot and turns the house over once every five minutes. The standard setting is 0.3 and turns it over every three minutes. The strongest setting is 0.375, which matches the Home Ventilating Institute rule of 3 CFM per square foot and turns the house over every two minutes.

Worked example: a 2,000 square foot home

A single-story home of 2,000 square feet with 8-foot ceilings, run at the standard setting.

• Volume of air: 2,000 x 8 = 16,000 cubic feet
• Standard factor: 0.3 CFM per cubic foot
• Airflow: 16,000 x 0.3 = 4,800 CFM
• Air changes: 4,800 x 60 / 16,000 = 18 an hour

For that fan you need an attic that can release the air. At one square foot of net free vent area per 750 CFM, a 4,800 CFM fan calls for 6.4 square feet of net free vent area. If your vents are covered with insect screen, which cuts the open area by about half, the total vent area you measure has to be closer to 12.8 square feet.

The house also needs somewhere for the air to come in. At 4 square feet of screened window per 1,000 CFM, a 4,800 CFM fan wants about 19.2 square feet of open window, which is two or three windows open a foot each.

Raise the setting to the strongest and the same home needs 6,000 CFM, 8 square feet of net free vent area and 24 square feet of open window. Drop to the gentle setting and it needs 3,200 CFM, 4.3 square feet of vent area and 12.8 square feet of window.

Venting is half the job: attic area and open windows

A whole house fan is an air mover, not an air maker. Every cubic foot it pushes into the attic has to leave through a roof or gable vent, and every cubic foot that leaves the house has to come in through an open window. Get either side wrong and the fan underperforms no matter how big it is.

The attic side follows a simple ratio. You need one square foot of net free vent area for every 750 CFM of airflow, which is the guideline the Department of Energy publishes. Net free area is the open space in a vent after you subtract the frame, the louvers and the screen. Insect screen alone can cut a vent's open area in half, so measure the vents you have and halve the number if they are screened.

The window side follows a second ratio: about 4 square feet of screened window per 1,000 CFM. Open windows in the rooms you want cooled first and on the shaded side of the house, because pulling in air that has been sitting in the sun defeats the point. Close windows in rooms you are not using so the breeze concentrates where it helps.

Two mistakes come up again and again. The first is running the fan with only one small window open, which starves it for air and makes it loud without moving much air. The second is an attic with ridge and soffit vents sized for normal attic breathing, which is nowhere near enough for a fan pushing thousands of CFM through it. Undersized attic venting is the single most common reason a whole house fan disappoints its owner.

CFM chart by home size

Airflow at the three settings, for homes with 8-foot ceilings. Multiply by ceiling height divided by 8 for taller rooms.

The HVI publishes a shorter version of the same table for its strongest recommendation, 3 CFM per square foot: 1,000 square feet needs 3,000 CFM, 2,000 needs 6,000 and 3,000 needs 9,000. If you want the fan to feel like a real breeze on a warm evening, aim at that top row. If you want a gentler turnover that uses less power and makes less noise, the standard row is the usual choice.

Whole house fan against air conditioning

A whole house fan does not cool air the way an air conditioner does. It replaces the hot air inside with cooler air from outside, and it cools the building materials, the walls, the floors and the furniture, so the house stays comfortable for a while after you switch the fan off.

That means the fan only helps when the outside air is cooler than the inside air, which in most of the country means early morning, late evening and overnight. Run it then, close the windows when the outside air warms up, and the house holds the cool for hours. On a 95 F afternoon the fan has nothing to work with and you want the air conditioner instead.

The running cost difference is large. A whole house fan draws 120 to 600 watts, so an hour of running costs about 2 to 5 cents at a typical electricity rate. A central air conditioner draws 3,000 to 5,000 watts, so an hour costs closer to 40 cents. In a climate with cool nights, a fan can cut air conditioning use substantially, and the Department of Energy notes that ventilation is the least expensive way to cool a home.

What this estimate leaves out

The calculation gives you a target airflow, and a few things can move the final choice. Climate is the biggest one. In a hot, humid climate a whole house fan pulls in damp air and can make the house feel sticky, so it matters less than in a dry climate with cool nights. In the Pacific Northwest or the mountain West, a fan often carries most of the summer cooling load on its own.

House tightness matters too. A tight modern home draws air in through fewer openings, so the fan works against more resistance and moves less air than its rating suggests. Old, leaky homes move more air than the rating but pull it through cracks rather than windows, which cools unevenly.

Noise is worth thinking about before you buy. Fans are rated in sones, and a large fan running at a low speed is usually quieter for the same airflow than a small fan running flat out. If the fan sits above a bedroom, look at the sone rating as well as the CFM.

Finally, this is a sizing estimate, not an installation plan. A whole house fan needs a properly framed and sealed ceiling opening, a cover for the winter so it does not leak heat, and enough attic venting to work. Confirm the size and the venting with the manufacturer or a qualified installer before you cut into the ceiling.

This is an estimate, not engineering or safety advice. Code requirements and product ratings change without notice. Confirm the final size with the manufacturer or a licensed installer.