Roof Ventilation Calculator

This calculator determines the net free area (NFA) of ventilation an attic needs, using the conservative 1/150 rule (total NFA equals attic floor area divided by 150), then splits that requirement 50/50 between intake and exhaust. Attic Floor Area drives NFA Required directly and proportionally -- double the attic and you double the required ventilation area. Roof Pitch adds an extra requirement on top of that baseline, but only once pitch exceeds 6:12; steeper roofs above that point add roughly 10% more required NFA for every additional pitch point, reflecting the larger attic volume a steep roof encloses.

Vent Type -- Ridge, Soffit, or Gable -- sets how much NFA each individual vent provides (18, 60, and 200 sq in respectively) for sizing Vents Needed, but it has no effect whatsoever on Total NFA Required, Intake NFA, or Exhaust NFA, since those three figures are set entirely by attic size and pitch before any vent-type-specific math happens. Balanced ventilation matters here: the calculator assumes intake and exhaust NFA should be equal, and adding exhaust vents without matching intake can actually depressurize the attic rather than improve airflow.