Roof Ventilation Calculator
Net free area for intake and exhaust vents.
Enter your values
Enter the attic floor area in sq ft.
Choose one of: 1:150 (no vapour barrier), 1:300 (balanced with barrier).
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
- 5 sq ft (720 sq in)
- Intake (soffit) area
- 360 sq in
- Exhaust (ridge) area
- 360 sq in
- Soffit vents needed
- 14 at 26 sq in each
- Ridge vent length
- 20 ft at 18 sq in per ft
Quick answer
Attics need 1 square foot of net free vent area per 150 square feet of attic floor, or per 300 with a vapour barrier and balanced venting.
NFA = attic area / ratio, split 50/50 intake and exhaust
At a glance
| What it does | Net free area for intake and exhaust vents. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Attic floor area, Code rule |
| Main output | Total net free area |
| Formula | NFA = attic area / ratio, split 50/50 intake and exhaust |
| Cost | Free — no sign-up, no download |
How to use the Roof Ventilation Calculator
- 1Enter the attic floor area in square feet.
- 2Choose 1:300 if you have a vapour barrier and balanced vents, otherwise 1:150.
- 3Split the area roughly evenly between low intake and high exhaust.
Inputs explained
- Attic floor area(sq ft)
- Enter the attic floor area in sq ft.
- Code rule
- Choose one of: 1:150 (no vapour barrier), 1:300 (balanced with barrier).
Worked example
Using the values the calculator loads with:
Inputs
- Attic floor area1500 sq ft
- Code rule1:300 (balanced with barrier)
Results
- Total net free area5 sq ft (720 sq in)
- Intake (soffit) area360 sq in
- Exhaust (ridge) area360 sq in
- Soffit vents needed14 at 26 sq in each
- Ridge vent length20 ft at 18 sq in per ft
Frequently asked questions
Why split intake and exhaust?
Air must enter low and leave high; exhaust-only venting pulls conditioned air out of the house instead.
Does vent net free area equal vent size?
No — louvres and screens block much of the opening, so use the manufacturer's NFA rating.
What do I need to enter into the Roof Ventilation Calculator?
Just 2 values: attic floor area, code rule. 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 Roof Ventilation Calculator work out the answer?
Attics need 1 square foot of net free vent area per 150 square feet of attic floor, or per 300 with a vapour barrier and balanced venting. It applies the formula NFA = attic area / ratio, split 50/50 intake and exhaust and shows the working so you can check each step by hand.
Is the Roof 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.