Roof Vent Calculator

The building code standard (1:150 rule, IRC R806.2) requires 1 sq ft of net free ventilation area (NFVA) for every 150 sq ft of attic floor space, split 50/50 between intake (soffit vents at the eaves) and exhaust (ridge or box vents high on the roof). A 1,500 sq ft attic needs 10 sq ft of NFVA, or 1,440 sq in — so 720 sq in on each side. On the exhaust side that is 43 linear feet of ridge vent (17 sq in of NFVA per foot) or 14 box vents (55 sq in each), and on the intake side 40 linear feet of soffit vent (18 sq in per foot). Only half the requirement is met by the exhaust vent you choose; the soffit intake is not optional.

Ridge vents with baffled soffit vents are the most effective passive ventilation system — they create a continuous airflow from eave to ridge. Box vents work well when ridge vents aren't possible. Power attic ventilators can over-ventilate and pull conditioned air from the living space if the attic is not properly sealed, so they should be used cautiously. Never mix ridge vents and power vents on the same attic.

Per unit installed, including labor: box vents $110–$270 each, ridge vent $11–$24 per linear foot, power attic ventilators $500–$1,050 per fan, and soffit vent $7–$17 per linear foot. On a 1,000 sq ft attic the exhaust side needs 29 linear feet of ridge vent, so ridge vent runs $319–$696. A full balanced system for a 2,000 sq ft attic — 57 linear feet of ridge vent for exhaust plus 54 linear feet of soffit vent for intake — runs about $1,005–$2,286. Building the same system out of box vents instead of a ridge vent costs more, roughly $2,358–$5,778, because you need 18 of them.

Yes — and if they are not equal, intake should be the larger of the two. IRC R806.2 caps upper (exhaust) ventilators at "not less than 40 percent and not more than 50 percent of the required ventilating area", with the balance at the eaves. The Asphalt Roofing Manufacturers Association states the same bands: "intake vents provide 50 to 60% of the open venting area and exhaust vents provide 40 to 50%". Air Vent Inc. puts the floor on the intake side: "Always install at least half, if not more, of the total required net free area needed for proper ventilation in the intake area." A 50/50 split is the one point that satisfies all three at once, and it is what this calculator sizes to. Under-sizing intake is the more common mistake, and it makes an exhaust vent pull conditioned air up out of the house instead of drawing fresh air in at the eaves.

Under-ventilation causes heat buildup in summer (raising cooling costs 10–15% and reducing shingle lifespan by up to 50%), moisture accumulation in winter (leading to mold, wood rot, and insulation damage), and ice dam formation in cold climates. Proper ventilation is one of the most important factors in roof longevity and home energy efficiency.