AtticVentCalc Tip

Attic ventilation calculator

Enter the attic floor length and width and AtticVentCalc applies the building-code ratio: 1 sq ft of net free vent area per 150 sq ft of attic, or per 300 sq ft with balanced high and low vents. It splits the area between intake at the soffits and exhaust near the ridge, then converts it into feet of ridge vent, box vents, feet of continuous soffit vent or individual soffit vents using each product's net free area. The example is a 40 × 30 ft attic: 8 sq ft, or 1,152 sq in.

Vent ratings (net free area)

    Saved you some time? Leave a tip, entirely optional.

    Open "Vent ratings" to enter the net free area printed on the vents you are buying.

    How the attic ventilation calculator works

    The International Residential Code (R806.2) asks for at least 1 sq ft of net free vent area for every 150 sq ft of attic floor. That drops to 1 per 300 when 40–50% of the vent area is in the upper part of the attic (within 3 ft of the ridge) with the rest at the eaves, or when there is a Class I or II vapor retarder on the warm side of a ceiling in cold climates. AtticVentCalc multiplies by 144 to get square inches, splits the total between intake and exhaust (50/50 by default; intake should never be less than exhaust), and divides by each vent's net free area rating: ridge vent 18 sq in per ft, box vents 50 sq in, continuous soffit strip 9 sq in per ft and 16 × 8 in soffit vents 56 sq in unless you change them.

    Vent area by attic size (50/50 split)

    Attic floor1/150 NFARidge ventContinuous soffit1/300 NFARidge vent
    600 sq ft576 sq in16 ft32 ft288 sq in8 ft
    900 sq ft864 sq in24 ft48 ft432 sq in12 ft
    1,200 sq ft1,152 sq in32 ft64 ft576 sq in16 ft
    1,500 sq ft1,440 sq in40 ft80 ft720 sq in20 ft
    1,800 sq ft1,728 sq in48 ft96 ft864 sq in24 ft
    2,400 sq ft2,304 sq in64 ft128 ft1,152 sq in32 ft

    Net free area is printed on the vent or its packaging and varies a lot between products, so check yours. Insulation baffles must keep soffit vents open, and local codes, roofing manufacturers' warranties and unvented (spray-foam) attic designs can change the requirement. Results are estimates for planning.

    Frequently asked questions

    How much attic ventilation do I need?

    At least 1 sq ft of net free area per 150 sq ft of attic floor, or 1 per 300 when the vents are balanced between the eaves and the ridge.

    What is net free area?

    The open area of a vent after screens and louvers, in square inches. It is printed on the vent and is always less than the vent's size.

    Should intake and exhaust be equal?

    Yes, or with more intake than exhaust. Too little intake lets exhaust vents pull air from the house or from each other.

    Is AtticVentCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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