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.
Quick answer: net free vent area at 1:150
| Attic floor | Required NFA | Ridge vent, half | Soffit vents, half |
|---|---|---|---|
| 800 ft² | 768 in² | 21.3 ft | 6 |
| 1,000 ft² | 960 in² | 26.7 ft | 8 |
| 1,200 ft² | 1,152 in² | 32 ft | 9 |
| 1,500 ft² | 1,440 in² | 40 ft | 12 |
| 2,000 ft² | 1,920 in² | 53.3 ft | 15 |
| 2,500 ft² | 2,400 in² | 66.7 ft | 19 |
IRC R806.2: 1 ft² of net free area per 150 ft² of attic, split half up and half at the eaves. Ridge vent at 18 in² per foot, soffit vents at 65 in² each (the calculator's defaults; use your product's NFA). Where 1:300 is allowed, halve everything.
| 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.
What I check on a job
Laying the vent count back on the roof. Before I trust the vent count, I lay it back onto the roof plan, because the page knows the attic's area and nothing about its shape. The ridge length it returns has to fit on the ridge that exists. On a hip roof the ridge can be short, and 40 feet of ridge vent on a ridge shorter than that means the scheme fails as drawn. I do not cover that shortfall with extra soffit vents when the job relies on 1:300: R806.2 wants 40 to 50 percent of the area up high, and shifting the balance to the eaves drops the upper share below that, back to 1:150. The soffit count gets the same test. Twelve vents have to land in rafter bays that open into the attic, not in bays closed off over a porch or a bay window. And the area I type is the space being ventilated, not the footprint, so a vaulted room under part of the roof comes out of it.
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.
Worked example
A 1,500 ft² attic under the IRC 1:150 rule: how much vent area, and how many vents?
- Net free area = 1,500 ÷ 150 = 10 ft² = 1,440 in².
- Balanced: 720 in² at the ridge, 720 in² at the eaves.
- Ridge vent rated 18 in²/ft: 720 ÷ 18 = 40 ft. Soffit vents at 65 in² each: ceil(720 ÷ 65) = 12.
1,440 in² of net free area: 40 ft of ridge vent and 12 soffit vents. At 1:300, where R806.2 permits it, the requirement halves to 720 in².
15 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How much attic ventilation do I need?
1/150 of the attic floor area in net free area under IRC R806.2: 10 ft² (1,440 in²) for a 1,500 ft² attic, split half at the ridge and half at the eaves.
When is 1/300 allowed?
With a Class 1 or 2 vapor retarder in climate zones 6–8, or when 40–50% of the vent area is in the upper attic and the rest at the eaves — the balanced ridge-and-soffit arrangement.
Is a vent's size its net free area?
No. A louver, a screen and a baffle each take part of the opening back: a 16 × 8 in. soffit vent is 128 in² of sheet metal and something like 65 in² of free area. Enter the manufacturer's NFA rating, per vent or per linear foot of ridge vent.
Why does the page put half the area at the ridge?
Because the 1/300 option in IRC R806.2 needs 40 to 50% of the required area in the upper attic, no more than 3 ft below the ridge, with the rest at the eaves. With more exhaust than intake, a ridge vent draws its make-up air from the nearest opening, which can be itself or the house.