Attic Ventilation Calculator
Attic vent NFA with the 1/150 or 1/300 rule, intake and exhaust.
About this tool
Enter the attic floor area, choose the rule, set the intake share, and enter the net free area printed on your soffit vents, ridge vent and box vents. The tool calculates the total, intake and exhaust NFA and how many vents or how much ridge vent you need.
How much attic ventilation?
A ventilated attic needs airflow to carry away moisture, which otherwise condenses and causes mould, rot and damp insulation, and to reduce summer heat. Codes express the requirement as net free area (NFA), the actual open area of vents after screens and louvres, as a fraction of the attic floor area. The basic rule is 1/150; it relaxes to 1/300 when there's a vapour retarder on the warm side of the ceiling or the ventilation is balanced between low intake vents (soffits or eaves) and high exhaust vents (ridge or roof vents). A 1,500 sq ft attic at 1/300 needs 5 sq ft, or 720 sq in, of NFA. Split it evenly, or with a little more intake (never less), and that's 360 sq in of soffit vents (40 vents of 9 sq in) plus 20 ft of ridge vent rated 18 sq in per foot. Honest limits: NFA ratings vary a lot between products, so use the figure printed on the vent. Don't combine different exhaust types such as ridge and gable vents, which can short-circuit airflow. Unvented 'hot roof' assemblies follow different rules. Check local building codes. Not building advice. The roofing calculator and the insulation calculator are related.
Frequently asked questions
What is the 1/300 rule for attic ventilation?
1 sq ft of net free vent area for every 300 sq ft of attic floor, split between intake and exhaust.
What is net free area (NFA)?
The unobstructed open area of a vent, after screens and louvres, usually printed on the product.
Should intake and exhaust be equal?
Balanced or slightly more intake is best; never have more exhaust than intake.
How many soffit vents do I need?
Divide the intake NFA by each vent's NFA. 360 sq in at 9 sq in per vent is 40 vents.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.