Gable Vent Size Calculator
Gable vent area for attic ventilation.
Enter your values
Enter the attic area in sq ft.
Choose one of: 1:300, 1:150.
Enter the number of gable vents you are working with.
Enter the louvre free area in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Gable vent area for attic ventilation. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Attic area, Ratio, Number of gable vents, Louvre free area |
| Main output | Total NFA required |
| Formula | vent NFA = attic area / ratio, split between gables |
| Cost | Free — no sign-up, no download |
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%.
Inputs explained
- Attic area(sq ft)
- Enter the attic area in sq ft.
- Ratio
- Choose one of: 1:300, 1:150.
- Number of gable vents
- Enter the number of gable vents you are working with.
- Louvre free area(%)
- Enter the louvre free area in %.
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.
What do I need to enter into the Gable Vent Size Calculator?
Just 4 values: attic area, ratio, number of gable vents, louvre free area. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Gable Vent Size Calculator work out the answer?
Gable vents supply attic exhaust area, sized from the same 1:150 or 1:300 net free area rule. It applies the formula vent NFA = attic area / ratio, split between gables and shows the working so you can check each step by hand.
Is the Gable Vent Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.