Roof Ventilation Net Free Area Calculator
Size attic intake and exhaust ventilation with the 1/300 net free area rule, then convert it into a vent count.
How it works
Required NFA (sq in) = attic area ÷ ratio × 144 · Intake = exhaust = NFA ÷ 2 · Vents = ceil(NFA share ÷ NFA per vent)
Attic ventilation is sized by net free area (NFA) — the actual open area of a vent after screens and louvers, not its outside dimensions. The building code rule is one square foot of NFA per 300 square feet of attic floor when intake and exhaust are balanced (1:150 if they are not). Splitting the requirement evenly, with intake low at the soffits and exhaust high at the ridge, drives a continuous flow that removes heat and moisture.
Worked example
A 1,500 sq ft attic at 1:300 needs 1,500 ÷ 300 = 5 sq ft of NFA, or 720 sq in. Half is intake (360 sq in) and half exhaust (360 sq in); at 9 sq in per soffit vent that is 40 intake vents, and at 50 sq in per exhaust vent, about 8 exhaust vents.
Frequently asked questions
What is net free area?
Net free area is the unobstructed opening a vent provides for airflow, measured in square inches. It is always smaller than the vent’s physical size because insect screens and louver blades block part of the opening — manufacturers publish the NFA for each product.
When can I use the 1:300 ratio instead of 1:150?
The 1:300 rule applies when at least 40% and no more than 50% of the ventilation is intake and it is balanced with exhaust, and often when a vapor retarder is present. Otherwise the code falls back to the more demanding 1:150 ratio.
Why split intake and exhaust evenly?
Balanced airflow needs roughly equal intake at the soffits and exhaust at the ridge so cool air is drawn in low and warm, moist air pushed out high. Too much exhaust without matching intake can pull air from the living space instead.
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This calculator is for educational purposes. Double-check important results before acting on them.