Enter your Attic Length, Attic Width, and Total Attic Square Footage, then choose your Building Code Standard, Primary Vent Type, and Roofing Material to calculate your Total Net Free Vent Area Required — plus the exact split between Intake Vents and Exhaust Vents Needed and your Ridge Vent Length Required.
Total Net Free Vent Area Required
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Intake Vents Needed (50%)
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Exhaust Vents Needed (50%)
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Ridge Vent Length Required
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The 1/300 rule requires 1 square foot of net free vent area for every 300 square feet of attic floor space. This is the standard building code requirement for most residential applications.
The 1/150 rule (double the ventilation) is required in high moisture areas, by some local building codes, or when vapor barriers are not properly installed. Always check your local building codes.
Proper attic ventilation requires 50% of the net free area at the intake (soffit/eave vents) and 50% at the exhaust (ridge or roof vents) to create effective airflow.
NFVA is the actual open area available for airflow after accounting for screens, louvers, and other obstructions. It's always less than the gross vent opening size.
While possible, it's generally not recommended to mix different exhaust vent types as they can interfere with each other's performance. Stick to one primary exhaust system.
Most ridge vents provide about 18 square inches of NFVA per linear foot. Divide your required exhaust area by 18 to get the linear feet of ridge vent needed.
Power vents move more air than passive vents, but they still need adequate intake ventilation to function properly. Follow manufacturer specifications for CFM ratings and intake requirements.