Attic vent area estimator

Roof Ventilation Calculator

Estimate total attic net free ventilation area, intake vent count, exhaust vent count, waste, and rough vent cost from attic floor area and product net-free-area rating. Use it as a planning check before verifying code and product rules.

Units

Attic floor area (ft²)Ventilation ratio denominator Intake share (%)Product NFA per vent (sq in)Allowance / balancing (%)Cost per vent ($)

Net free area

Intake vents

Exhaust vents

Rough cost

Ventilation is code and climate specific

This calculator uses simple NFA ratios for planning. Local code, vapor control, insulation, balanced intake and exhaust, bath fans, ridge details, baffles, roof geometry, and product ratings control the final design.

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Roof ventilation formula

Total NFA sq in = attic floor area ft² × 144 ÷ ventilation ratio

Intake NFA = total NFA × intake split

Exhaust NFA = total NFA - intake NFA

Vent count = NFA share × (1 + waste %) ÷ product NFA

Sample takeoff

A 1,200 ft² attic using a 1/300 ratio, 50/50 intake and exhaust split, 50 sq in vent products, and 10% allowance needs about 576 sq in of total NFA and seven intake plus seven exhaust vents.A 110 m² attic using a 1/300 ratio, 50/50 intake and exhaust split, 325 cm² vent products, and 10% allowance needs about 3,667 cm² of total NFA and seven intake plus seven exhaust vents.

Roof Ventilation Calculator FAQ

How do I calculate attic ventilation area?+

Divide attic floor area by the ventilation ratio, convert to product NFA units, then split the area between intake and exhaust.Divide attic floor area by the ventilation ratio, convert to product NFA units, then split the area between intake and exhaust.

What is NFA?+

NFA means net free area: the open vent area after louvers, mesh, and product restrictions. Use the NFA printed by the vent manufacturer.NFA means net free area: the open vent area after louvers, mesh, and product restrictions. Use the NFA printed by the vent manufacturer.

Does this prove code compliance?+

No. Local code, climate, vapor control, insulation, and balanced intake and exhaust rules decide the final ventilation design.No. Local code, climate, vapor control, insulation, and balanced intake and exhaust rules decide the final ventilation design.