Attic Ventilation Calculator

Calculate proper attic ventilation requirements. Balance intake and exhaust for optimal airflow and shingle warranty compliance.

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Calculator

Attic Floor Area *

sq ft

Square footage of attic floor space

Climate Zone *

Intake Vent Type *

Exhaust Vent Type *

Existing Intake NFVA

sq in

Existing ventilation (if any)

Existing Exhaust NFVA

sq in

Existing ventilation (if any)

Attic Ventilation Requirements (FHA 1:300 Rule)


                    Required NFVA = Attic Sq Ft ÷ 300

Base ventilation requirement


                    Intake = Required × 0.60

60% should be low intake


                    Exhaust = Required × 0.40

40% should be high exhaust

The FHA 1:300 rule requires 1 square foot of net free vent area (NFVA) for every 300 square feet of attic floor space. This is split 60% intake (low/soffit) and 40% exhaust (high/ridge).

Cold climates: Increase to 1:150 ratio to prevent ice dams.
Hot climates: Consider 1:150 for better heat removal.

Net Free Vent Area (NFVA) is the actual open area for air flow, accounting for screens and louvers. Always use NFVA ratings, not physical dimensions.

Reference: FHA 1:300 Rule, International Residential Code (IRC) R806