A powered attic ventilator (PAV), usually called an attic fan, is a roof- or gable-mounted fan that pushes hot air out of the attic. The Home Ventilating Institute (HVI) sizes it to change the attic air at least 10 times an hour, which works out to 0.7 CFM per square foot of attic floor, plus about 15% for a dark or steep roof. The fan also needs enough soffit intake: 1 sq ft of net free area for every 300 CFM. The calculator and chart below give both numbers, and compare them with the passive vent area an attic needs anyway.
Attic fan size calculator
Attic fan CFM and soffit vent chart
| Attic floor area | Fan CFM (x 0.7) | Dark or steep roof (+15%) | Soffit intake, sq in (standard) | Soffit intake, sq in (+15%) | For comparison: passive vents at 1/150, sq in |
|---|---|---|---|---|---|
| 800 sq ft | 560 | 644 | 269 | 309 | 768 |
| 1,000 sq ft | 700 | 805 | 336 | 386 | 960 |
| 1,200 sq ft | 840 | 966 | 403 | 464 | 1,152 |
| 1,500 sq ft | 1,050 | 1,208 | 504 | 580 | 1,440 |
| 1,800 sq ft | 1,260 | 1,449 | 605 | 696 | 1,728 |
| 2,000 sq ft | 1,400 | 1,610 | 672 | 773 | 1,920 |
| 2,500 sq ft | 1,750 | 2,012 | 840 | 966 | 2,400 |
| 3,000 sq ft | 2,100 | 2,415 | 1,008 | 1,159 | 2,880 |
HVI’s own table matches: 1,000 sq ft of attic needs 700 CFM, or 805 CFM with the 15% allowance, and an 805 CFM fan needs 386 sq in of soffit intake. The 0.7 multiplier is 10 air changes an hour turned into a per-square-foot figure: it corresponds to an attic averaging about 4.2 ft high (0.7 x 60 / 10). A tall attic under a steep roof holds more air than that; for one, the volume in cubic feet x 10 / 60 gives the CFM for 10 air changes directly.
Intake is the limit
An attic fan cannot blow out more air than can get in. HVI requires at least 1 sq ft of inlet area for every 300 CFM of certified fan capacity and says to use only soffit vents as the intake: gable vents near the fan can let it pull rain and snow into the attic, and they short-circuit the airflow so the fan ventilates only the end of the attic. When the soffits are blocked by insulation, painted over or simply too small, the fan looks for air elsewhere, and the easiest source is the house below: through gaps around ceiling lights, the attic hatch, plumbing stacks and wiring holes. That pulls conditioned air out of the house, which is the opposite of what an attic fan is meant to do in summer.
Two habits avoid that. Seal the attic floor before installing a powered fan, and check that the soffit vents are clear, with baffles holding the insulation back at the eaves. The intake figure in the calculator is the minimum net free area; vent labels give the net free area per vent or per foot.
Powered or passive ventilation
Most attics are ventilated passively: warm air rises and leaves through vents at or near the ridge, and cooler air enters at the soffits. HVI recommends sizing static vents at 1 sq ft of net free area per 150 sq ft of attic, with about 60% of it low at the eaves and 40% high at the ridge or gables. That is the same 1/150 ratio the building code uses, and our attic ventilation calculator works it out with ridge and soffit vent counts.
A powered fan is an addition, not a substitute: it still needs the soffit intake. If you add one to an attic that already has a ridge vent, the fan may pull air in through the ridge vent next to it instead of through the soffits, leaving most of the attic unventilated; choose one or the other for the exhaust. A thermostat on the fan keeps it running only when the attic is hot, and a humidistat can run it when moisture is the concern.
Worked example: 1,500 sq ft attic, dark shingles
Fan: 1,500 x 0.7 = 1,050 CFM, plus 15% for the dark roof = 1,208 CFM. Soffit intake: 1,208 / 300 x 144 = 580 sq in of net free area. With soffit vents rated at 9 sq in each, that is 65 vents; with continuous soffit vent rated at 9 sq in per foot, about 65 ft of it, split along both eaves. Ratings vary by product, so use the number on the label.
Building codes also regulate exhaust fans, duct termination and makeup air, and they vary by state and city. HVI’s figures are industry guidance; your building department decides what is required on your job.
Related calculators
- Attic ventilation calculator (passive vents, net free area)
- Whole house fan size calculator: cooling the house through the attic
- Roofing calculator: squares, bundles and underlayment
Frequently asked questions
What size attic fan do I need?
Multiply the attic floor area by 0.7 for the CFM, which gives about 10 air changes an hour (HVI). Add 15% for a dark or steep roof. A 1,500 sq ft attic needs about 1,050 CFM, or 1,208 CFM with the allowance.
How much soffit vent do I need for an attic fan?
At least 1 sq ft of net free intake area for every 300 CFM of fan, or CFM / 300 x 144 sq in (HVI). A 1,050 CFM fan needs 504 sq in.
Can I use gable vents as intake for an attic fan?
HVI says to use only soffit vents as the intake for powered attic ventilators; gable vents can let the fan draw rain and snow into the attic.
Should I install an attic fan with a ridge vent?
Generally not both: a powered fan near a ridge vent can pull air in through the ridge instead of the soffits. Use the ridge vent with soffit intake, or the fan with soffit intake.
How many air changes per hour should an attic fan give?
At least 10, according to HVI, which is the basis of the 0.7 CFM per sq ft rule.
Sources
- Home Ventilating Institute (HVI), How Much Ventilation Do I Need? (HVI ventilation guide articles; read 25 October 2026). Powered attic ventilators (10 air changes per hour, x 0.7, +15%, soffit intake CFM / 300, soffit vents only) and static attic ventilation (1/150, 60/40 split).
Last reviewed: 25 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.