Roof Ventilation Calculator
Building codes size attic ventilation by “net free area” — the open area of the vents after screens and louvers. Enter your attic floor area and the NFA printed on your vents, and the calculator applies the IRC 1/150 rule (or 1/300 where an exception applies), splits it between intake and exhaust and tells you how many vents or how many feet of ridge vent to add.
Roof Ventilation Calculator
Calculate how much net free ventilating area (NFA) your attic needs, how to split it between intake and exhaust, and how many vents to add.
Your results appear here and update as you change the numbers.
How this calculator works
- Required NFA (sq ft) = attic floor area ÷ 150, or ÷ 300 when an IRC R806.2 exception applies. Multiply by 144 for square inches.
- 1/300 is allowed when (a) 40–50% of the required area is in vents in the upper part of the attic, no more than 3 ft below the ridge, with the rest at the eaves, or (b) in climate zones 6–8, a Class I or II vapor retarder is installed on the warm side of the ceiling.
- The split defaults to 50% exhaust and 50% intake; intake should never be less than exhaust.
- Vents to add = (required NFA − existing NFA) ÷ NFA per vent, rounded up.
Worked example
A 1,500 sq ft attic at 1/150 needs 1,500 ÷ 150 = 10 sq ft = 1,440 sq in of NFA: 720 sq in of intake and 720 of exhaust. With 50 sq in soffit vents: 720 ÷ 50 = 14.4 → 15 vents. With ridge vent at 18 sq in per foot: 720 ÷ 18 = 40 ft of ridge vent.
When to get a professional check
Your local building code and the roofing manufacturer’s warranty requirements control. Don’t mix exhaust types (for example ridge vent plus gable fans) without professional advice, and never block soffit intake with insulation.
Assumptions used — example values, not yet reviewed against current local prices
| Assumption | Value |
|---|---|
| Share of NFA placed as exhaust (high) | 50% |
Sources and references
Ready for real prices? Learn how to find and vet a local roofer.
How to choose a contractorResults are planning estimates, not quotes. Prices vary by location, contractor and the condition of your roof. Verify sizes and code requirements locally before buying or building.
Frequently asked questions
When can I use 1/300 instead of 1/150?
Under IRC R806.2, when 40–50 % of the vent area is high (within 3 ft of the ridge) with the rest at the eaves, or in climate zones 6–8 with a Class I or II vapor retarder on the warm side of the ceiling.
Should intake equal exhaust?
Balanced is the goal; intake should never be less than exhaust.
Can I mix ridge vents and gable fans?
Generally no — mixed exhaust can short-circuit airflow. Ask a pro.
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