Attic Fan CFM Calculator
Powered attic ventilator size and intake area.
Enter your values
ⓘ Live results
Results
- Fan size
- 1,300 CFM
- Base requirement
- 980 CFM
- Intake vent free area
- 622 sq in
- Air changes per hour
- 14 ACH (4 ft average height)
Quick answer
Attic fans are sized at about 0.7 CFM per square foot of attic floor, with extra for dark or steep roofs.
CFM = attic area x 0.7 x roof factor
How to use the Attic Fan CFM Calculator
- 1Enter the attic floor area, usually the same as the top-floor footprint.
- 2Select the roof pitch and colour, which both raise the heat load.
- 3Provide at least the intake free area shown, or the fan will pull air from the house.
Worked example
Using the values the calculator loads with:
Inputs
- Attic floor area1400 sq ft
- Roof pitchMedium (4:12–8:12)
- Roof colourDark
Results
- Fan size1,300 CFM
- Base requirement980 CFM
- Intake vent free area622 sq in
- Air changes per hour14 ACH (4 ft average height)
Frequently asked questions
Do attic fans save money?
They can lower attic temperature, but without adequate soffit intake they pull conditioned air out of the home and cost more than they save.
Passive or powered?
Ridge and soffit venting is usually enough; powered fans suit hot climates with poor passive airflow.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.