Net free area, often abbreviated NFA, is the actual open, unobstructed area of a vent through which air can pass, measured in square inches, as opposed to the vent's overall physical size. A louvered or screened vent's louvers and mesh block off part of its total opening, so its net free area is always smaller than its outer dimensions suggest. Building codes and ventilation calculations are based on net free area specifically, which is why two vents of the same visible size can provide very different amounts of actual airflow.
A vent's physical size and how much air it actually lets through are two different numbers, and net free area is the one that matters for ventilation calculations. It is the portion of a vent's opening that is genuinely unobstructed, measured in square inches, after accounting for whatever louvers, baffles, or insect screen sit across it.
A plain rectangular hole would have a net free area equal to its full physical opening, but almost no vent is built that way. Louvers are angled across the opening to keep out wind-driven rain, and a fine mesh screen keeps out insects and small animals, and both of those features physically block a portion of the total opening they sit across.
That is why manufacturers publish a specific net free area rating for every vent product, printed in square inches, rather than letting installers estimate it from the vent's outer dimensions. Two vents that look nearly identical in size on a shelf can have meaningfully different net free area ratings depending on their louver design and screen density.
Building codes and manufacturer ventilation guidelines calculate required ventilation in terms of net free area per square foot of attic space, not simply a vent count, which means the math behind a properly ventilated attic depends on adding up the actual rated NFA of every intake and exhaust vent installed, not just counting the number of vents on the roof.
A roof can appear to have generous ventilation, plenty of vents dotted across it, and still fall short of code or manufacturer requirements if the vents installed carry a lower net free area rating than the job actually calls for.
When sizing or troubleshooting attic ventilation, the specific NFA rating of the products involved is the number that actually settles whether the system is adequate, not a visual impression of how many vents sit on the roof.
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