Understanding your result
The headline is the airflow in CFM, with the fan size and the intake it needs. The stats repeat the fan size and the intake; the breakdown shows the attic volume, the airflow and the intake area.
The number not to skip is the intake: deliver it with the soffit vent calculator. If the attic already has passive exhaust from the ridge vent calculator or the box vent calculator, a fan is usually not needed; the attic ventilation calculator shows whether the passive vents meet the code.
How we calculate this
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Air changes per hour | 10 | Common target in attic fan makers’ sizing guides |
| Quick rule | 0.7 CFM per sq ft | Attic fan makers’ rule of thumb |
| Dark roof | + 15% | Makers’ guidance |
| Intake | 1 sq ft per 300 CFM | Attic fan makers’ installation guides |
| Fan sizes | 800 to 2,000 CFM | Common residential models |
Assumptions last reviewed October 7, 2026.
The calculator does not check ceiling air-sealing or combustion appliances in the attic. The ventilation guide discusses passive ventilation first.
Two worked examples
By attic volume
1,500 square feet, 32-foot span, 6/12, 10 air changes.
- Height: 8 ft; volume: 1,500 × 8 ÷ 2 = 6,000 cu ft
- CFM: 6,000 × 10 ÷ 60 = 1,000; intake 480 sq in
By the quick rule, dark roof
2,000 square feet, 0.7 CFM per square foot, dark shingles.
- CFM: 2,000 × 0.7 × 1.15 = 1,610; buy a 2,000 CFM fan
- Intake: 773 sq in
Where to find your inputs
Attic floor area, span and pitch. From the house; the pitch from the pitch calculator.
Air changes. Ten unless the fan maker says otherwise.
Common mistakes
- No intake. The fan pulls from the house.
- Fan plus ridge vent. Short circuit.
- Combustion appliances. Backdraft risk; have them checked.
- Unsealed ceiling. Seal first.
- Oversizing. More CFM needs more intake.
- No thermostat. Run it only when the attic is hot.
Questions people ask
- What size attic fan do I need?
- Enough airflow to change the attic air about 10 times an hour. A 1,500 square foot attic under a 6/12 roof on a 32-foot span holds about 6,000 cubic feet, so 6,000 × 10 ÷ 60 = 1,000 CFM. The quick rule of 0.7 CFM per square foot of attic floor gives 1,050 CFM for the same attic.
- How much intake does an attic fan need?
- About 1 square foot of net free intake per 300 CFM: 3.3 square feet, 480 square inches, for a 1,000 CFM fan. Without it the fan pulls air from the house.
- Do attic fans save energy?
- The evidence is mixed. A fan can cool a hot attic, but in an air-conditioned house it can also pull cooled air through ceiling leaks, cancelling the saving. A sealed ceiling with balanced passive ventilation is the usual recommendation; a fan is a fix for specific problems.
- Can I use an attic fan with a ridge vent?
- No. The fan pulls air in through the ridge vent instead of the soffits, short-circuiting the attic. Close the ridge vent near the fan or use passive ventilation only.
- Solar or wired attic fan?
- Solar fans run only when the sun is up, which is when the attic is hot, and need no wiring; wired fans move more air and run on a thermostat. Size both by CFM.
- Does a dark roof need a bigger fan?
- Dark shingles run hotter, so manufacturers suggest about 15 percent more airflow. The calculator adds it when ticked.