Roof Ventilation Calculator
Calculate the ventilation your attic needs based on building code requirements
Determine ventilation requirements
Quick presets
sq ft
Total NFA Required
5.0 sq ft
720 sq inches • 1:300 ratio
PRO
Professional Calculator
Full attic ventilation calculation per IRC R806
sq ft
Estimated Materials
0 bundles
Roof Area
0 sq ft
Squares
0.0
Detailed Breakdown
Roof Area0 sq ft
With Waste0 sq ft
Roofing Squares0.0
Bundles0
How to Use This Calculator
Enter your attic floor area in square feet and indicate whether a vapor barrier is installed. The calculator determines the total NFA (Net Free Area) required, then splits it 50/50 between intake and exhaust ventilation. It also calculates specific vent counts and ridge vent length based on your measurements.
The Formula
Total NFA Required = Attic Area ÷ 300 (with vapor barrier) or ÷ 150 (without)
Intake NFA = Total NFA ÷ 2
Exhaust NFA = Total NFA ÷ 2
Ridge Vent Length = Exhaust NFA ÷ NFA per linear foot (typically 18 sq in/ft)
Soffit Vents = Intake NFA ÷ NFA per vent (typically 50-80 sq in each)
Example Calculation
For a 1,500 sq ft attic with vapor barrier:
• Total NFA: 1,500 ÷ 300 = 5 sq ft = 720 sq inches
• Intake NFA: 360 sq in → 6 soffit vents (at 65 sq in NFA each)
• Exhaust NFA: 360 sq in → 20 ft of ridge vent (at 18 sq in NFA/ft)
• Total NFA: 1,500 ÷ 300 = 5 sq ft = 720 sq inches
• Intake NFA: 360 sq in → 6 soffit vents (at 65 sq in NFA each)
• Exhaust NFA: 360 sq in → 20 ft of ridge vent (at 18 sq in NFA/ft)
Frequently Asked Questions
How much attic ventilation do I need?
The FHA standard requires 1 sq ft of Net Free Area (NFA) for every 300 sq ft of attic floor space when a vapor barrier is present (1:300 ratio), or 1 sq ft for every 150 sq ft without a vapor barrier (1:150 ratio). Ventilation should be split 50/50 between intake (soffit) and exhaust (ridge/gable vents).
What is NFA in roofing?
NFA stands for Net Free Area — the actual open area of a vent that allows air to pass through. It's always less than the vent's physical size because of screens, louvers, and baffles. A vent with 50 sq inches of total area might have only 25 sq inches of NFA.
Can you have too much roof ventilation?
While rare, over-ventilation can be problematic in extreme climates. The main risk is allowing too much moisture-laden air into the attic in humid climates, or allowing wind-driven rain/snow to enter. Following the 1:300 or 1:150 code requirements is the right target.