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Attic Ventilation Area Calculator

Required net free vent area from attic floor area.

Compute the required net free vent area (NFA) for an attic using either the 1-to-150 ratio (no vapor barrier) or the 1-to-300 ratio (with vapor barrier or balanced ridge/soffit), per the IRC.

Marcus Laurent By Marcus Laurent
Published Last reviewed 1 min read

Inputs

ft²

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

Required NFA = floor_area_ft² / ratio; ratio = 150 or 300.

In depth

Compute the required net free vent area (NFA) for an attic using either the 1-to-150 ratio (no vapor barrier) or the 1-to-300 ratio (with vapor barrier or balanced ridge/soffit), per the IRC.