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Soffit Vent Calculator

Calculate soffit and ridge vent area for balanced attic ventilation. Enter attic floor area for IRC 1/150 or 1/300 rules with intake and exhaust balance.

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Inputs that matter
Attic Floor Area, Ventilation Type, Vent NFA Each
Output to expect
Soffit Vent Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Total NFA Required

IRC requires 1 sq ft NFA per 150 sq ft attic floor (1/150 rule). With balanced system and vapor retarder, 1/300 may apply.

NFA (sq in) = Attic Area (sq ft) × 144 ÷ 150 (or ÷ 300 with vapor barrier)

Intake and Exhaust Split

Balanced ventilation splits NFA equally between soffit intake and ridge or roof exhaust. Unbalanced systems reduce effectiveness.

Intake NFA = Exhaust NFA = Total NFA ÷ 2

Vent Unit Count

Divide required NFA by manufacturer-rated NFA per vent strip, panel, or continuous vent product.

Units = ceil(Required NFA ÷ NFA per Vent)

Updated: July 2026

Example Scenarios

Need 1,440 sq in total NFA — 720 sq in soffit intake and 720 sq in ridge exhaust. Continuous ridge vent plus perforated soffit panels.

576 sq in total NFA at 1/150. Smaller attic still needs proportional vent area — do not skip ventilation on attached garages.

Measure existing soffit NFA from vent area. Match ridge vent NFA to at least 50% of total required, not less than existing intake.

FAQ

1/150 applies by default: 1 sq ft vent area per 150 sq ft attic. 1/300 allowed when vents are balanced 50/50 high/low and ceiling has vapor barrier.

Exhaust-only ventilation pulls air from conditioned space through ceiling leaks. Adequate soffit intake ensures airflow path through attic, not from living space.

Gable vents work but may not provide even airflow across full attic length. Ridge + soffit is preferred for uniform ventilation on long roofs.

Manufacturers publish net free area per linear foot or per unit on packaging. Continuous soffit vent strip NFA adds along total perforated length.

Yes — air chutes (baffles) at eaves keep insulation from blocking soffit intake path. Install baffles before adding blown insulation.

About Soffit Vent Calculator

Size soffit intake and ridge exhaust ventilation for code-compliant attic airflow. Enter attic floor area and vent product NFA (net free area) to get required vent counts and verify 50/50 intake-exhaust balance.