Roof Ventilation Calculator
Net free area for intake and exhaust vents.
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ⓘ Live results
Results
- 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
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.
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.
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.