Attic ventilation is sized from net free ventilating area, not vent face dimensions. Establish the code or design ratio that applies, convert the attic area to required NFA, then balance intake and exhaust and divide by each selected vent product's published NFA.
Convert the attic floor area to required net free ventilating area under the applicable ratio, then divide the total between intake and exhaust and size products from their published NFA ratings.
Published: September 20, 2026
Last reviewed: September 20, 2026
Published by BuildYard. The guide reflects BuildYard's calculation methodology and editorial standards and does not replace project-specific professional advice.
Use the horizontal attic floor or ceiling area served by the ventilation system.
Separate isolated attic compartments when air cannot move freely between them.
Ventilation ratios can depend on code provisions and assembly details.
Do not assume a reduced ratio applies without confirming its conditions.
Translate the required fraction of attic area into square inches of NFA for product selection.
NFA is the open airflow rating after screens or louvers, not the gross vent opening.
A balanced system normally provides intake low at the eaves and exhaust high near the ridge.
Use the roof-system and local-code guidance controlling the project.
Divide required intake and exhaust NFA separately by the published rating of each chosen vent.
Round to complete vents or full ridge-vent lengths and verify unobstructed airflow paths.
BuildYard arithmetic example. GAF notes that some local codes require the 1/150 rule and its calculator reports intake and exhaust NFA separately.
| Step | Result |
|---|---|
| Attic area | 900 sq ft |
| Total NFA at 1/150 | 6 sq ft / 864 sq in |
| Balanced intake | 432 sq in |
| Balanced exhaust | 432 sq in |
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Estimate ridge vent length from required exhaust NFA, usable ridge length, product NFA per linear foot, and balanced intake capacity.
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