Attic Fan Intake Vent Compatibility Calculator
Check whether your soffit and ridge vents can supply enough intake air for your attic fan's CFM rating. Matching intake CFM to the exhaust CFM prevents negative pressure in the attic, which can pull conditioned air out of your house interior, which in turn pulls unconditioned outside air into your living space, leading to higher cooling costs.
Not sure what CFM your fan needs to be in the first place? Start with the CFM sizing calculator, then come back here with that number.
From the fan's spec sheet, or the sizing calculator above
Soffit Vents
Typical soffit panels run 16"×8" or 16"×4". Check your actual vent size — undercounting here is the most common source of error.
Ridge Vent
Your intake venting supplies about 533 CFM, short of the 1,580 CFM this fan needs. That gap can pull conditioned air out of your house to make up the difference (undesirable).
You need about 6.98 more sq ft of Net Free Area. At your current soffit vent size, that's roughly 16 more soffit panels, or about 56 more linear feet of ridge vent if you add that instead.
Suggested Fan
Even our smallest listed fan needs more intake than your current venting supplies. Add soffit or ridge venting first, or this fan will still pull conditioned air from the house.
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How This Is Calculated
Net Free Area (NFA) is the actual open area of a vent, not its physical dimensions. A standard aluminum louver vent is roughly 50% NFA; a full-open mesh vent is roughly 75% NFA.
Ridge vent intake is estimated at roughly 18 sq in of NFA per linear foot, though this varies by manufacturer — check your product's spec sheet for an exact figure.
Intake capacity uses the rule of thumb of 1 sq ft of NFA per 150 CFM of fan capacity.
This is a sizing estimate, not an energy audit. Actual airflow depends on vent placement, obstructions like insulation baffles, and how well-sealed the attic floor is. When in doubt, oversize the intake — excess intake capacity doesn't cause the negative-pressure problem that undersized intake does.