The ratio uses net free area, not the outside size of the vents.
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
What is the 1/300 rule?
Half the 1/150 requirement, allowed when the vents are balanced high and low (40–50% at the top) or when a vapor retarder is installed in cold climates.
Is the attic area the floor or the roof area?
The attic floor (ceiling) area, the footprint of the space below the roof.
How many square inches is 1/150 for 1,500 sq ft?
1,500 ÷ 150 = 10 sq ft = 1,440 sq in, split 720 intake and 720 exhaust.
Is AtticVentCalc free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.