Ridge Vent Airflow Calculator
Passive exhaust airflow through a ridge vent by stack effect.
Enter your values
ⓘ Live results
Results
- 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)
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.
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.
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.