What does a soffit vent calculation chart show for my attic size?
Short answer. A soffit vent chart starts from the code's balanced ratio: attic floor area times 0.48 gives total net free area in square inches, and about half of that, attic area times 0.24, is the minimum soffit intake. A 2,000 square foot attic, for example, needs at least about 480 square inches of soffit intake, which you then divide by your vent's rated net free area.
Soffit intake chart (balanced system)
Feet of strip vent below assume a product rated at 9 square inches per foot; substitute your product's rating.
- 1,000 sq ft attic: 240 sq in of intake, about 27 feet of strip vent.
- 1,500 sq ft: 360 sq in, about 40 feet.
- 2,000 sq ft: 480 sq in, about 54 feet.
- 2,500 sq ft: 600 sq in, about 67 feet.
- 3,000 sq ft: 720 sq in, about 80 feet.
- 3,500 sq ft: 840 sq in, about 94 feet.
How to use it
- Measure the attic floor area, not the roof surface.
- Find the intake figure for that size.
- Divide by your vent's rating per foot or per vent.
- Spread the result along all eaves. A two-sided gable roof needing 54 feet gets about 27 feet per side.
Adjustments
- If exhaust is larger than this intake figure, raise intake to at least match it.
- An unbalanced system, using the 1/150 ratio, needs double these numbers.
- Do not count soffit over porches or rooms that do not connect to the attic.
- Vents buried in insulation do not count until baffles open them up.
Individual vents
For rectangular undereave vents, use the net free area stamped on the vent, not its face size, and expect to need more of them than you might think. See roof ventilation basics.