Attic Ventilation Calculator
Net free vent area from the IRC R806.2 ratio, split between eave intake and ridge exhaust, turned into a ridge length and a soffit vent count.
Net free vent area from the IRC 1:150 rule — or 1:300 where the code lets you — split between eave intake and ridge exhaust, then turned into a count of the vents you are actually buying.
| Required net free area | — |
|---|---|
| Exhaust (upper), half | — |
| Intake (eaves), half | — |
| Ridge vent length | — |
| Soffit vents | — |
One square foot of opening for every 150 of attic. IRC R806.2 sets the minimum net free ventilating area at 1/150 of the area of the space being ventilated. The 1/300 alternative is allowed in two cases the section spells out: a Class 1 or 2 vapor retarder on the warm‑in‑winter side of the ceiling in climate zones 6, 7 and 8, or at least 40% and not more than 50% of the required area located in the upper part of the attic (no more than 3 feet below the ridge) with the rest at the eaves. That second condition is what a balanced ridge‑and‑soffit system does, which is why the page splits the area in half.
Net free area is not the size of the hole. A louver, a screen and a baffle each take part of the opening back, and the number a vent is sold on is the manufacturer’s NFA after all of them. A 16×8‑inch soffit vent is 128 square inches of sheet metal and something like 65 of free area; a ridge vent is rated per linear foot. Both inputs above are those ratings, and they come from the product, not from here.
Balanced means intake at least equals exhaust. The upper vents are limited to half the requirement for a reason: with more exhaust than intake, a ridge vent pulls its make‑up air from the nearest opening, which can be the other side of the same ridge vent, or the house. If the soffits are blocked by insulation the arithmetic here is satisfied and the attic still does not breathe; baffles at the eaves are part of the job.
Required NFA = attic area ÷ 150 (or ÷ 300 where IRC R806.2 permits), converted to square inches. Exhaust and intake are each half of that; ridge length = exhaust ÷ NFA per foot; soffit count = ceil(intake ÷ NFA each). Vent NFA figures are the manufacturer’s ratings. The rule is IRC 2021 §R806.2 “Minimum vent area”; the vent‑placement condition for 1/300 is R806.2 item 2.