Gable Vent Size Calculator
Gable vent area for attic ventilation.
Enter your values
ⓘ Live results
Results
- Total NFA required
- 576 sq in
- Gross area per vent
- 480 sq in
- Square vent size
- 21.9 x 21.9 in
- Per vent NFA
- 288 sq in
- Reminder
- Pair with equal soffit intake
Quick answer
Gable vents supply attic exhaust area, sized from the same 1:150 or 1:300 net free area rule.
vent NFA = attic area / ratio, split between gables
How to use the Gable Vent Size Calculator
- 1Measure the attic footprint and choose your code ratio.
- 2Divide the area between the number of gable vents.
- 3Adjust for the louvre free area — screens and blades block roughly 40%.
Worked example
Using the values the calculator loads with:
Inputs
- Attic area1200 sq ft
- Ratio1:300
- Number of gable vents2
- Louvre free area60 %
Results
- Total NFA required576 sq in
- Gross area per vent480 sq in
- Square vent size21.9 x 21.9 in
- Per vent NFA288 sq in
- ReminderPair with equal soffit intake
Frequently asked questions
Gable and ridge together?
It can short-circuit airflow; most roofers pick one exhaust strategy.
Do gable vents let rain in?
Properly louvred vents with screens shed rain, but exposed walls benefit from a hooded design.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.