Roof Vent Calculator
Good attic ventilation keeps the roof deck dry, reduces ice dams and condensation, and helps shingles last. Enter your attic floor area to see how much vent area you need and how to split it between intake and exhaust.
Last reviewed September 2026
- Intake (soffit / low)
- 384 sq in
- Exhaust (ridge / high)
- 384 sq in
Codes require 40–50% of the area as high exhaust; an even split is the usual target. Net free area per vent is printed on each product. Local code and roof design can change the requirement.
How the calculation works
Divide the attic floor area by 150 (or 300 where allowed) to get the required net free area in square feet, then multiply by 144 for square inches. Split it roughly evenly: intake vents low at the eaves or soffits bring in cool air, and exhaust vents high near the ridge let warm, moist air out. Divide each half by the net free area printed on the vent you plan to use.
Signs your attic needs more ventilation include frost on nails in winter, very hot attic air in summer and repeated ice dams. See attic condensation and ice dams.
Attic ventilation questions
What is the 1/300 rule for attic ventilation?
Building codes require 1 square foot of net free vent area for every 150 square feet of attic floor. That can drop to 1 per 300 when conditions are met, typically when 40–50% of the vent area is high exhaust venting and the rest is low intake venting (or a vapor retarder is used in cold climates).
What is net free area (NFA)?
It's the open area a vent actually lets air through, after screens and louvers. Every vent product lists its NFA, usually in square inches per vent or per linear foot.
Can an attic have too much ventilation?
Mixing different exhaust types, such as a ridge vent with gable vents or powered fans, can short-circuit airflow so the attic doesn't ventilate properly. Balance intake and exhaust rather than just adding more vents.