Change the box vent rating under "Vent ratings" to match the product you are using.
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 floor | 1/150 NFA | Ridge vent | Continuous soffit | 1/300 NFA | Ridge vent |
|---|---|---|---|---|---|
| 600 sq ft | 576 sq in | 16 ft | 32 ft | 288 sq in | 8 ft |
| 900 sq ft | 864 sq in | 24 ft | 48 ft | 432 sq in | 12 ft |
| 1,200 sq ft | 1,152 sq in | 32 ft | 64 ft | 576 sq in | 16 ft |
| 1,500 sq ft | 1,440 sq in | 40 ft | 80 ft | 720 sq in | 20 ft |
| 1,800 sq ft | 1,728 sq in | 48 ft | 96 ft | 864 sq in | 24 ft |
| 2,400 sq ft | 2,304 sq in | 64 ft | 128 ft | 1,152 sq in | 32 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 many box vents per square foot of attic?
About one 50 sq in box vent per 100 sq ft of attic at the 1/150 rule with a 50/50 split, or one per 200 sq ft at 1/300.
Where should box vents go?
Close to the ridge (within about 3 ft of it), spaced evenly, with soffit vents providing intake below.
Do I need more box vents or a ridge vent?
Either can supply the exhaust area; ridge vent spreads it along the whole roof and is often preferred on gable roofs.
Is AtticVentCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.