Attic Ventilation Calculator
Net free area from the two rules everyone quotes — 1:150 and 1:300 — split evenly between intake and exhaust, and turned into ridge feet and vent counts.
The two standard ratios, computed. The conditions on the 1:300 rule (balanced venting, sealed ceiling plane with vapor retarder) and any local amendments are the building department’s call — this page guarantees no compliance outcome.
How the math works
Net free area (NFA) = attic floor area ÷ the rule ratio: 1 sq ft of vent per 150 sq ft of attic, or per 300. The 1:300 rule is the conditional one — it applies when the venting is balanced between ridge and soffit and the ceiling carries a vapor retarder/air-sealed plane; when in doubt, 1:150 is the conservative classic. Half the NFA belongs at the intake (soffits) and half at the exhaust (ridge/roof) — a ridge vent without soffit intake is decoration.
Ridge feet = exhaust sq in ÷ your ridge product’s NFA per foot (18 sq in/ft is a common shingle-over ridge figure). Soffit count = intake sq in ÷ the vent’s stamped NFA. Both product numbers are inputs — the stamp on the product wins.
A worked example
A 1,500 sq ft attic at 1:300: 5.0 sq ft of NFA = 720 sq in total, 360 in and 360 out. That is 20 ft of ridge vent at 18 sq in/ft, and 8 soffit vents at 50 sq in each. At 1:150 the same attic needs exactly double.
Questions from the field
How much attic ventilation do I need? The classic rule is 1 sq ft of net free vent area per 150 sq ft of attic floor — or per 300 when intake and exhaust are balanced and the ceiling plane is sealed with a vapor retarder. This page computes both so the choice is explicit.
What is net free area? The actual open area air moves through after screens and louvers take their cut. It is stamped on vent products as NFA — a vent’s physical size is always bigger than its NFA.
Why split intake and exhaust evenly? Exhaust-only venting pulls air from wherever it can — including your living space through ceiling leaks. Balanced intake at the soffits is what makes the ridge vent breathe the attic instead of the house.
Does this guarantee code compliance? No. It runs the two standard ratios. Local amendments and the conditions attached to the 1:300 rule are the building department’s call.
The other calculators
Want us to look at your shop?
The free tools cover the leaks most shops share. The free Snapshot is the one-on-one version: 15 free minutes with our team to pressure-test what you found and map your next step. No pressure either way — you leave with a short list you can act on yourself.
Book Your Free Snapshot — book your free 15-minute call first, then answer a few quick questions, so the call itself is spent on your shop, not on background.