What Size Attic Fan Do I Need? CFM by Attic Size and Intake
A power attic fan should move about 0.7 CFM for every square foot of attic floor, or 0.805 CFM if the roof is dark or steep. A 50 × 30 ft attic (1,500 ft²) needs 1,050 CFM, or 1,208 CFM with a dark roof. It also needs 504 in² of soffit intake, since the rule is 1 ft² of intake for every 300 CFM. The attic fan size calculator does the sums for your attic. These are the sizing rules published by the Home Ventilating Institute.
Part of the project planners: Planning a roof replacement and Planning heating, cooling and ventilation.
Fan size by attic area
CFM for a standard roof and a dark or steep roof, with the soffit intake the standard fan needs:
| Attic floor | Standard roof | Dark or steep roof | Intake (in²) | Intake (ft²) |
|---|---|---|---|---|
| 800 ft² | 560 CFM | 644 CFM | 269 in² | 1.87 ft² |
| 1,000 ft² | 700 CFM | 805 CFM | 336 in² | 2.33 ft² |
| 1,200 ft² | 840 CFM | 966 CFM | 403 in² | 2.80 ft² |
| 1,500 ft² | 1,050 CFM | 1,208 CFM | 504 in² | 3.50 ft² |
| 1,800 ft² | 1,260 CFM | 1,449 CFM | 605 in² | 4.20 ft² |
| 2,000 ft² | 1,400 CFM | 1,610 CFM | 672 in² | 4.67 ft² |
| 2,500 ft² | 1,750 CFM | 2,012 CFM | 840 in² | 5.83 ft² |
By attic length and width
A standard roof, with the number of 1,200 CFM fans:
| Attic | Area | CFM needed | Fans of 1,200 CFM |
|---|---|---|---|
| 40 × 20 ft | 800 ft² | 560 CFM | 1 |
| 40 × 30 ft | 1,200 ft² | 840 CFM | 1 |
| 50 × 30 ft | 1,500 ft² | 1,050 CFM | 1 |
| 60 × 30 ft | 1,800 ft² | 1,260 CFM | 2 |
| 60 × 40 ft | 2,400 ft² | 1,680 CFM | 2 |
Before you install
- Check the intake first: add soffit vents, or clear blocked ones, before the fan. Use the attic ventilation calculator to size the soffits.
- A fan with too little intake pulls air from the house through gaps around lights and the hatch. Seal the attic floor.
- With a gas furnace or water heater in the house, ask a pro to check for backdrafting.
- Use a thermostat so the fan runs only when the attic is hot, at about 100 to 110 degrees F.
- Do not mix a power fan with a ridge vent or gable vents: choose one exhaust system.
- More attic insulation keeps the heat out of the rooms below: see the attic insulation calculator.
Estimates only: have the electrical connection made by a qualified electrician where code requires.
Frequently asked questions
What size attic fan do I need?
0.7 CFM for every square foot of attic floor. A 1,500 ft² attic needs 1,050 CFM, or 1,208 CFM if the roof is dark or steep (0.805 CFM per square foot).
How much intake vent does an attic fan need?
One square foot of net free soffit vent area for every 300 CFM. For 1,050 CFM that is 504 in² (3.50 ft²).
Is a bigger attic fan better?
No. A fan bigger than the intake can supply pulls conditioned air from the house through the ceiling, and can backdraft a gas furnace or water heater.
Can I use an attic fan and a ridge vent together?
It is best not to. The fan can pull air in through the ridge vent instead of the soffits, so the attic is not ventilated evenly. Choose one exhaust system.
What CFM is a solar attic fan?
Most are rated at about 800 to 1,600 CFM in full sun. Use the same sizing rule, and note that the rating falls on cloudy days.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.