Attic Ventilation Calculator

IRC R806.2 net free area in square inches — 1:150 default, 1:300 only when the exception qualifies, split into intake and exhaust

This is airflow and code compliance, not a shingle takeoff. Squares, bundles, and underlayment live in the Roofing Square & Shingle Bundle Calculator. Why attics need balanced intake/exhaust: guide. Moisture, ice dams, heat: troubleshooting. Ridge vs turbines vs gable: comparison. Roofing index: roofing calculators hub. Eave drainage (not NFA): gutter & downspout sizing calculator. Ice dams from attic heat: ice & water shield coverage calculator. Weight of the pack itself: roof snow load calculator.

Calculator Inputs

Attic floor area of the vented space, vapor-retarder and climate-zone checks from IRC R806.2, and a 50/50 intake/exhaust split (40% high is the other code-legal option). Default ratio is 1:150. 1:300 applies only when the exception conditions are actually met.

Attic floor area (sq ft, vented space)

Use the attic floor (or the floor of the rafter space you are venting), not the house living-area listing and not the sloped roof surface from the shingle square calculator. Those are different numbers.

Vapor retarder on the warm-in-winter side of the ceiling?

No Yes (Class I or II)

IRC R806.2 Exception item 1: in Climate Zones 6, 7, and 8 a Class I or II vapor retarder must be on the warm-in-winter side of the ceiling. Polyethylene is Class I; kraft-faced batts facing the room are typically Class II. In Zones 1–5 this vapor-retarder item does not apply.

Climate zone

Zones 1–5 Zones 6, 7, or 8

The vapor-retarder condition is only a 1:300 gate in Zones 6–8. In milder zones, a balanced 40–50% high layout alone unlocks 1:300. Confirm your IECC climate zone; do not guess from a ZIP you have not looked up.

Intake / exhaust split

50/50 (standard) 40% high / 60% low

Shown because the code cares. Exception item 2 requires not less than 40% and not more than 50% of the required area high (within 3 ft of the ridge, measured vertically), with the balance in the bottom third (soffits / eaves). 50/50 is the usual target. 40% exhaust / 60% intake is the other edge of that window. More than 50% high fails the exception.

Results

Total net free area required

—

Enter attic floor area, vapor retarder, and climate zone

Ratio used—

Code sectionIRC R806.2

NFA (sq ft)—

Intake NFA (low)—

Exhaust NFA (high)—

Split—

If 1:150 (always allowed)—

If 1:300 (exception only)—

IRC R806.2 (2021/2024): minimum net free ventilating area is 1/150 of the vented space. The 1/300 exception requires both listed conditions — balanced 40–50% high vents, and in Climate Zones 6–8 a Class I or II vapor retarder on the warm-in-winter side of the ceiling. This tool returns total NFA in square inches, not a carton count. Soffit, ridge, gable, and turbine units each have their own NFA stamp. Unvented attics are R806.5. Local amendments and shingle warranties can be stricter. Why attics need this: guide. Moisture and ice dams: troubleshooting. Exhaust types: ridge vs turbines vs gable.

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How to Use This Attic Ventilation Calculator

Enter the attic floor area of the space you are venting. Say whether a Class I or II vapor retarder is on the warm-in-winter side of the ceiling, pick Climate Zones 6–8 vs 1–5, and keep the split at 50/50 unless you are deliberately running 40% high. The tool applies IRC R806.2: 1:150 unless the 1:300 exception actually qualifies, then splits net free area into intake (low) and exhaust (high) in square inches.

NFA (sq ft) = attic floor area ÷ ratio
ratio = 150, or 300 only if R806.2 Exception both conditions are met
NFA (sq in) = NFA (sq ft) × 144
exhaust = NFA × 0.40 to 0.50 intake = the balance

This is airflow / code compliance, not a shingle takeoff. Squares and bundles live in the Roofing Square & Shingle Bundle Calculator. How to read an NFA stamp: guide. Moisture, ice dams, cooked shingles: troubleshooting. Ridge vs turbines vs gable: comparison.

What This Number Is (and Is Not)

Net free area is the actual opening after screens, louvers, and weather baffles. IRC R806.2 sizes the attic with that number, not the rectangle you cut in the soffit. A 16×8 undereave vent is not 128 sq in of NFA; Air Vent’s 84210 lists 56. A ridge slot is not the plywood gap; ShingleVent II and GAF Cobra Rigid Vent 3 list 18 sq in per linear foot.

Assumptions baked into every result

One vented attic, one climate-zone pick, one vapor-retarder yes/no. Split is 50/50 or 40% high / 60% low — both sit inside the 40–50% high window. The 1:300 path requires that window and, in Zones 6–8, the vapor retarder. High vents are assumed within 3 ft of the ridge. Unvented / spray-foam attics under R806.5 are out of scope. Product count is out of scope on purpose: NFA per piece varies by SKU.

Folk rule vs the actual exception.Older talk said “vapor barrier = 1:300.” The current IRC exception is both: balanced high/low, and the vapor retarder only as a cold-climate gate. This calculator follows the published R806.2 text, not the shortcut.

When this tool is the wrong tool

Shingle squares, bundles, and felt rolls belong in the roofing square calculator. Common rafter length belongs in the rafter length calculator. Powered attic fans, bathroom-fan soffit caps, and whole-house fans are not R806.2 gravity ventilation.

Already have the roof area in squares?That number is surface, not attic floor. Measure the attic floor (or the ceiling below it) and bring that square footage here.

Open Square Calculator →

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Attic Ventilation Gear

The calculator returns net free area in square inches, not a carton count. A soffit intake, a ridge exhaust, and a hygrometer are how you turn that NFA into a balanced attic and then check that it is actually dry.

Air Vent 84210 16x8 in Aluminum Undereave Soffit Vent

16 x 8 in undereave intake with 56 sq in net free area per piece. The calculator gives total intake NFA; divide by the stamp on the vent you actually buy.

View on Amazon →

GAF Cobra Exhaust Vent 10.5 in x 20 ft Mesh Roll Ridge Vent

Shingle-over ridge exhaust. GAF Cobra Rigid Vent 3 lists 18 sq in NFA per linear foot — confirm the roll or stick you buy. Exhaust NFA from the calculator, not a SKU count.

View on Amazon →

ThermoPro TP50 Digital Indoor Thermometer and Hygrometer

Temperature plus relative humidity with high/low memory. Hang it in the attic after you size NFA — moisture on rafters is a ventilation symptom, not a square-count problem.

View on Amazon →

Run the NFA Count

Attic floor area, vapor retarder, climate zone, and 50/50 split. Total, intake, and exhaust net free area in square inches.

Open Attic Ventilation Calculator →

Need Squares, Not Airflow?

Shingle bundles and underlayment from footprint and pitch. Materials takeoff — not IRC ventilation NFA.

Open Roofing Square Calculator →

Roofing Calculators Hub

Squares and bundles plus attic ventilation on one index. Framing pitch still lives on the rafter calculator.

Open Roofing Hub →

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Attic Ventilation Calculator

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How to Use This Attic Ventilation Calculator