Free Calculator
Attic Ventilation Calculator
Enter the attic square footage to get the net free area you need under the 1/300 and 1/150 rules, split into balanced soffit intake and ridge exhaust — plus the approximate linear feet of each vent.
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Size Attic Intake and Exhaust
Enter the attic floor area, choose the ventilation ratio, and see the balanced intake and exhaust net free area you need — in square inches and linear feet of vent.
Usually equals the home's footprint under the attic.
Use 1/150 if there's no vapor barrier.
50% is balanced. Never under 50% intake.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates for planning purposes only. Actual attic ventilation requirements depend on local building code, roof and attic geometry, and the rated net free area of the specific vents you install. Always confirm against local code and the vent manufacturer's specifications before ordering material.
How Attic Ventilation Sizing Works
The 1/300 and 1/150 rules
Attic ventilation is sized off the attic floor area. The two accepted ratios are:
| Rule | NFA required | When to use it |
|---|---|---|
| 1/300 | 1 sq ft per 300 sq ft of attic | Balanced 50/50 intake/exhaust with a vapor barrier |
| 1/150 | 1 sq ft per 150 sq ft of attic | No vapor barrier or unbalanced airflow — the safer target |
Multiply the resulting square feet of NFA by 144 to get square inches, which is how vents are rated.
Balance intake and exhaust
Ventilation only works when it moves air. Split the total NFA roughly 50% intake low at the soffits and 50% exhaust high near the ridge so warm, moist air rises out the top while fresh air is pulled in at the eaves. If exhaust exceeds intake, a ridge or power vent starts pulling conditioned air out of the living space or drawing weather in. When you can't hit exactly 50/50, lean toward more intake, never more exhaust.
Why net free area — not vent count — is what matters
Net free area is the actual open area air can pass through after subtracting louvers, screens, and baffles. A vent's physical size overstates how much air it moves, so ventilation is always sized by adding up rated NFA in square inches. This calculator converts required NFA into approximate linear feet of continuous ridge and soffit vent using typical ratings (about 18 sq in/ft for ridge and 9 sq in/ft for soffit), but the rating on the actual product always wins.
Why ventilation belongs on every roof estimate
Under-ventilated attics trap heat and moisture, which shortens shingle life, drives up cooling bills, and invites mold and ice dams — problems that come back on the roofer who installed the system. Pricing intake and exhaust into every re-roof protects the workmanship warranty and gives the homeowner a longer-lasting roof. If you run a roofing crew, standardizing scopes and material takeoffs in roofing contractor software keeps ventilation from getting dropped off the estimate.
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Book a Free DemoFrequently Asked Questions
How much attic ventilation do I need?
The standard guideline is the 1/300 rule: you need 1 square foot of net free ventilation area (NFA) for every 300 square feet of attic floor space, split evenly between intake (low, at the soffits) and exhaust (high, at or near the ridge). If the attic has no vapor barrier or the intake/exhaust split is not balanced, the more conservative 1/150 rule applies — 1 square foot of NFA per 150 square feet of attic. This calculator applies both rules and converts the result into square inches and approximate lengths of ridge and soffit vent.
What is net free area (NFA)?
Net free area is the actual open area of a vent that air can pass through, after subtracting the material of louvers, screens, and baffles. It's almost always smaller than the vent's physical size, and it's the number that matters for ventilation math. Every code-rated vent lists its NFA in square inches, so you size ventilation by adding up NFA — not by counting vents or measuring their outside dimensions.
What is the 1/300 rule for attic ventilation?
The 1/300 rule means 1 square foot of net free ventilation area for every 300 square feet of attic floor area, with a balanced split of roughly 50% intake at the soffits and 50% exhaust high on the roof. It's the ventilation ratio most building codes accept when the attic has a proper vapor barrier and balanced airflow. Without those conditions, the 1/150 rule (twice the ventilation) is the safer target.
Should attic intake and exhaust ventilation be balanced?
Yes. Balanced ventilation — roughly 50% intake low at the soffits and 50% exhaust high near the ridge — is what creates the continuous convective airflow that carries heat and moisture out of the attic. If exhaust exceeds intake, ridge or power vents can pull conditioned air from the living space or draw weather in through other openings. When in doubt, err toward slightly more intake than exhaust, never the reverse.
Can I mix different types of attic exhaust vents?
Generally no — mixing exhaust types on the same attic (for example a ridge vent plus box vents, or a ridge vent plus a powered fan) often short-circuits the airflow, so one exhaust pulls air from the other instead of from the intake. Pick one exhaust type per attic space and pair it with enough soffit intake. Intake vents of different styles can usually be combined as long as the total intake NFA is met.
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Get a Free DemoAll results are approximate planning figures and should be verified against local building code and the vent manufacturer's net-free-area specifications before ordering material.