Attic Fan CFM Sizing Calculator
Find the minimum and recommended CFM for your attic calculated based on floor area and climate, then check it against popular gable and solar attic fan models. This calculator helps you optimize energy savings and efficiency by avoiding an oversized fan that wastes energy and requires more intake venting than necessary to cool down the attic.
Sizing the fan is only half the job — without enough intake venting to match, the fan pulls conditioned air out of your house instead of just exhausting the attic. Once you have a target CFM, run it through the intake vent compatibility calculator to confirm your soffit and ridge venting can keep up.
Total square footage of your attic floor
Recommended Fan For This Attic
The smallest fan on the list that still comfortably covers your attic — enough CFM without paying for, or having to vent, more than you need.
All Fans Compared
"Oversized" means well beyond this attic's recommended range — more cost and noise than needed, and a bigger intake vent requirement to avoid depressurizing the attic.
How This Is Calculated
Standard climate: 0.7 CFM per square foot of attic floor area.
Hot climate (Florida, Arizona, Texas, and similar): 1.0 CFM per square foot, since the attic absorbs more heat for more hours per day.
Recommended range adds 15% headroom above the minimum to account for real-world derating — duct resistance, insect screens, and static pressure all reduce a fan's output below its free-air CFM rating.
This is a sizing rule of thumb, not a substitute for a manufacturer's engineering guidance for unusual roof geometry or multiple attic sections. It also assumes your intake venting can supply enough replacement air — check that separately with the intake vent compatibility calculator.