This attic ventilation calculator sizes required net free area (NFA) for your attic by the building-code 1/150 rule (and the 1/300 exception), split evenly between soffit intake and ridge/roof exhaust.
| Total NFA required | – |
| Intake (soffit) — 50% | – |
| Exhaust (ridge/roof) — 50% | – |
| Ridge vent length at 18 sq in NFA/ft | – |
NFA is the vent’s actual open area after screens and louvers — a “100 sq in” louvered vent often nets half that. Buy by the NFA printed on the product, never by the hole size, and keep intake ≥ exhaust so the attic draws from soffits rather than from your living space.
Hot, Wet Attic Air Is a Roof Killer — Ventilation Is the Drain
The other half of attic performance is our Insulation Calculator. Part of our Construction Calculators collection.
Worked Example (Verified)
- 1,200 sq ft attic, 1/150 rule (defaults): total NFA = 8.00 sq ft = 1,152 sq in → 576 sq in intake + 576 exhaust; as ridge vent at 18 sq in/ft that’s 32 linear ft of ridge — a number most 1960s gable-vent houses miss by half
- The 1/300 path (ceiling vapor barrier + balanced high/low vents, where adopted): halves everything — 576 sq in total; know your jurisdiction before banking the discount
- Blocked soffits quietly zero the intake side: insulation baffles at every rafter bay are the $2 part that makes the whole system real
Ventilation Discipline
- Balance matters more than totals: excess exhaust without intake pulls conditioned air (and moisture) from the house through ceiling leaks.
- Never mix exhaust types at the ridge (ridge vent + powered fan + turbines fight each other, short-circuiting the soffit draw).
- Bath fans and dryer vents must terminate outside — an attic “vented” by a bath fan is a mold incubator wearing a code costume.
- Unvented (conditioned) attics are a legitimate modern alternative with foam at the roofline — a different system entirely; don’t half-mix the two philosophies.
Related Tools
- Insulation Calculator — R-value & coverage
- Roofing Calculator — the roof above the attic
- Construction Calculators — the full collection
Frequently Asked Questions
How much attic ventilation do I need?
Code standard: net free area = attic floor area ÷ 150. A 1,200 sq ft attic needs 8 sq ft (1,152 sq in) of NFA, split evenly between low intake (soffits) and high exhaust (ridge/roof). With a ceiling vapor barrier and balanced vents, many codes allow 1/300 — half as much. This calculator runs both rules.
What is net free area (NFA)?
The actual open area of a vent after its screen, mesh and louvers take their cut — often 50–75% of the vent’s face size. Products are rated in NFA (sq in); always size by that number, and prefer vents with published NFA over ones sold by rough hole dimensions.
Why split intake and exhaust evenly?
Stack-effect flow needs an entrance matched to the exit: soffit air sweeps the underside of the deck and leaves at the ridge. Intake-starved systems pull air from the conditioned house instead (energy loss, moisture transport), while exhaust-starved systems stagnate no matter how big the soffits are.
How many feet of ridge vent do I need?
Divide the exhaust NFA by your product’s NFA per linear foot (a common rating is 18 sq in/ft): the defaults’ 576 sq in exhaust → 32 ft. If your ridge is shorter than the requirement, supplement with roof vents rated in NFA — added, not substituted blindly.
Does attic ventilation really extend roof life?
Yes: summer attic heat (140°F+ unvented) cooks shingles from below and voids some warranties, while winter moisture from the house condenses on cold decking and rots it. Ventilation moderates both — it’s moisture management in winter, heat management in summer.
Is this attic ventilation calculator free?
Yes — free, instant, no sign-up.