Ridge Vent Airflow Calculator
Passive exhaust airflow through a ridge vent by stack effect.
Enter your values
Enter the ridge vent net free area in sq ft.
Enter the vertical vent separation (ridge to soffit) in ft.
Enter the attic-to-outside temp diff in °F.
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.
- Estimated passive airflow
- 1,138 CFM
- Net free area used
- 2 sq ft
- Note
- Actual airflow also depends on wind, which typically dominates over stack effect
- Balance check
- Soffit intake NFA should equal or exceed ridge NFA
Quick answer
Ridge vent airflow depends on net free area and the stack effect from attic-to-outside temperature difference; well-designed systems commonly move several hundred CFM passively in warm attics.
CFM ~ NFA(sq ft) x C x sqrt(height x deltaT)
At a glance
| What it does | Passive exhaust airflow through a ridge vent by stack effect. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Ridge vent net free area, Vertical vent separation (ridge to soffit), Attic-to-outside temp diff |
| Main output | Estimated passive airflow |
| Formula | CFM ~ NFA(sq ft) x C x sqrt(height x deltaT) |
| Cost | Free — no sign-up, no download |
How to use the Ridge Vent Airflow Calculator
- 1Get the ridge vent's rated net free area (from the product data, in sq ft).
- 2Enter the vertical separation between ridge and soffit vents.
- 3Enter a typical attic-to-outside temperature difference to estimate stack-driven airflow.
Inputs explained
- Ridge vent net free area(sq ft)
- Enter the ridge vent net free area in sq ft.
- Vertical vent separation (ridge to soffit)(ft)
- Enter the vertical vent separation (ridge to soffit) in ft.
- Attic-to-outside temp diff(°F)
- Enter the attic-to-outside temp diff in °F.
Worked example
Using the values the calculator loads with:
Inputs
- Ridge vent net free area2 sq ft
- Vertical vent separation (ridge to soffit)10 ft
- Attic-to-outside temp diff25 °F
Results
- Estimated passive airflow1,138 CFM
- Net free area used2 sq ft
- NoteActual airflow also depends on wind, which typically dominates over stack effect
- Balance checkSoffit intake NFA should equal or exceed ridge NFA
Frequently asked questions
Does wind matter more than stack effect?
Often yes — ridge vents are primarily wind-driven, with stack effect providing a baseline even in calm conditions.
What if soffit intake is undersized?
The ridge vent will starve for makeup air and airflow drops sharply — balance soffit and ridge NFA 1:1.
What do I need to enter into the Ridge Vent Airflow Calculator?
Just 3 values: ridge vent net free area, vertical vent separation (ridge to soffit), attic-to-outside temp diff. 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 Ridge Vent Airflow Calculator work out the answer?
Ridge vent airflow depends on net free area and the stack effect from attic-to-outside temperature difference; well-designed systems commonly move several hundred CFM passively in warm attics. It applies the formula CFM ~ NFA(sq ft) x C x sqrt(height x deltaT) and shows the working so you can check each step by hand.
Is the Ridge Vent Airflow 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.