Attic Ventilation Calculator
Net free vent area, intake and exhaust split for any attic.
Enter your values
Enter the attic floor area in sq ft.
Choose one of: 1/150 (no vapour barrier), 1/300 (balanced + vapour barrier).
Enter the nfa per soffit vent in sq in.
Enter the nfa per foot of ridge vent in sq in/ft.
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 net free area
- 4.67 sq ft (672 sq in)
- Intake required
- 336 sq in
- Exhaust required
- 336 sq in
- Soffit vents needed
- 8
- Ridge vent length
- 18.67 ft
Working
- 11,400 / 300 = 4.67 sq ft NFA, half each side
Quick answer
Attics need 1 sq ft of net free vent area per 150 sq ft of attic floor (or 1 per 300 with a vapour barrier), split evenly between intake and exhaust.
NFA = attic area / ratio, split 50% intake and 50% exhaust
At a glance
| What it does | Net free vent area, intake and exhaust split for any attic. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Attic floor area, Code ratio, NFA per soffit vent, NFA per foot of ridge vent |
| Main output | Total net free area |
| Formula | NFA = attic area / ratio, split 50% intake and 50% exhaust |
| Cost | Free — no sign-up, no download |
How to use the Attic Ventilation Calculator
- 1Measure the attic floor area, not the roof surface.
- 2Use 1/300 only if you have a proper ceiling vapour barrier and balanced vents.
- 3Match intake to exhaust - too much ridge vent with too little soffit pulls air from the house.
Inputs explained
- Attic floor area(sq ft)
- Enter the attic floor area in sq ft.
- Code ratio
- Choose one of: 1/150 (no vapour barrier), 1/300 (balanced + vapour barrier).
- NFA per soffit vent(sq in)
- Enter the nfa per soffit vent in sq in.
- NFA per foot of ridge vent(sq in/ft)
- Enter the nfa per foot of ridge vent in sq in/ft.
Worked example
Using the values the calculator loads with:
Inputs
- Attic floor area1400 sq ft
- Code ratio1/300 (balanced + vapour barrier)
- NFA per soffit vent45 sq in
- NFA per foot of ridge vent18 sq in/ft
Results
- Total net free area4.67 sq ft (672 sq in)
- Intake required336 sq in
- Exhaust required336 sq in
- Soffit vents needed8
- Ridge vent length18.67 ft
- 1,400 / 300 = 4.67 sq ft NFA, half each side
Frequently asked questions
What is net free area?
The actual open area of a vent after louvres and insect screen, always smaller than its physical size. Manufacturers publish it on the box.
Can I have too much exhaust?
Yes. If ridge exhaust exceeds soffit intake, the vent starts drawing conditioned air out of the living space, which wastes energy and can cause moisture problems.
What do I need to enter into the Attic Ventilation Calculator?
Just 4 values: attic floor area, code ratio, nfa per soffit vent, nfa per foot of ridge vent. 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 Attic Ventilation Calculator work out the answer?
Attics need 1 sq ft of net free vent area per 150 sq ft of attic floor (or 1 per 300 with a vapour barrier), split evenly between intake and exhaust. It applies the formula NFA = attic area / ratio, split 50% intake and 50% exhaust and shows the working so you can check each step by hand.
Is the Attic Ventilation 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.