Attic Ventilation Calculator

Divide the attic floor area by 300 when intake and exhaust are balanced. A 1,500 sq ft attic needs 5 sq ft of net free area, or 720 square inches — about 432 in² of soffit intake and 288 in² of high exhaust. If the vents are not balanced, or no vapor retarder is installed where the code requires one, the ratio drops to 1:150 and you need double: 10 sq ft, or 1,440 square inches. This is a planning estimate; your local building department has the final say.

Net free area (NFA) is the actual open area air can pass through after the louvers, insect screen, and frame are subtracted — not the size of the hole you cut. A 16 × 8 inch soffit vent has a 128 square inch gross opening but typically only 45 to 65 square inches of NFA. Every vent is stamped or spec-sheeted with its rating, and you must size from that number. Painting over a soffit vent or stuffing insulation behind it destroys the NFA you paid for.

Intake should always equal or exceed exhaust, and a 60/40 intake-heavy split is the common target — that is what this calculator uses. The IRC exception that allows the 1:300 ratio requires 40 to 50 percent of the area to be in the upper portion of the attic, at least 3 feet above the eave vents, with the balance at the eaves. Exhaust-heavy attics pull make-up air backward through other roof vents or from the conditioned space below instead of from outside.

No. A powered fan moves far more air than the soffits can supply, so it pulls make-up air down through the ridge vent or the nearest roof vents instead of from the eaves — the attic barely ventilates and the fan can even depressurize the space and draw conditioned air through ceiling leaks. Most manufacturers say to use one exhaust type per attic space. If you install a ridge vent, remove or seal off box vents, gable fans, and turbines on that same space.

Yes. IRC Section R806 requires a minimum of 1 square foot of net free ventilating area per 150 square feet of vented space, with an exception permitting 1 per 300 when the vents are balanced as described above or a Class I or II vapor retarder is installed on the warm side in colder climate zones. Openings must be screened against pests with mesh between 1/16 and 1/4 inch. Amendments vary by jurisdiction, so treat every number here as a planning estimate only — your local building code governs, and a licensed roofing contractor or your inspector should confirm the vent layout before you cut anything.