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Attic Ventilation Net Free Area Calculator
Attic ventilation sizing uses net free vent area, not the grille’s outside dimensions. Divide attic floor area by the selected design ratio, convert square feet to square inches, then allocate intake and exhaust. This calculator is a planning aid only; applicable codes, vapour control, climate, roof geometry, insulation, fire requirements, and manufacturer ratings determine the correct system.
Planning estimate only. Check measurements and real-world constraints before buying materials or making a commitment.
Calculate your scenario
Change any input. Results update immediately.
Your results
Required total net free area
768 in²
At the entered floor-area ratio.
Additional intake vent units
5
Based on entered NFA per unit.
Additional exhaust vent units
6
Balanced against the selected intake share.
How the calculation works
The calculator applies this relationship to the inputs above. Keep every measurement in the unit shown.
Worked example
Use this example to check the calculator by hand before relying on a result.
Assumptions behind the result
- • Attic is one connected ventilation zone.
- • Selected ratio is applicable.
- • Existing vents are unblocked.
- • Manufacturer NFA is known.
- • Mechanical and unvented roof designs are evaluated separately.
Mistakes that change the answer
- • Using vent face dimensions.
- • Adding exhaust without intake.
- • Blocking soffit pathways with insulation.