Attic Ventilation Calculator
Net free vent area, intake and exhaust split for any attic.
Enter your values
ⓘ Live results
- 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
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.
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.
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.