Attic Fan CFM Calculator
Powered attic ventilator size and intake area.
Enter your values
Enter the attic floor area in sq ft.
Choose one of: Low (under 4:12), Medium (4:12–8:12), Steep (over 8:12).
Choose one of: Light, Dark.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Powered attic ventilator size and intake area. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Attic floor area, Roof pitch, Roof colour |
| Main output | Fan size |
| Formula | CFM = attic area x 0.7 x roof factor |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Attic floor area(sq ft)
- Enter the attic floor area in sq ft.
- Roof pitch
- Choose one of: Low (under 4:12), Medium (4:12–8:12), Steep (over 8:12).
- Roof colour
- Choose one of: Light, Dark.
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.
What do I need to enter into the Attic Fan CFM Calculator?
Just 3 values: attic floor area, roof pitch, roof colour. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Attic Fan CFM Calculator work out the answer?
Attic fans are sized at about 0.7 CFM per square foot of attic floor, with extra for dark or steep roofs. It applies the formula CFM = attic area x 0.7 x roof factor and shows the working so you can check each step by hand.
Is the Attic Fan CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.