How it works
The calculator turns the attic floor area into a required net free area, sizes the exhaust first and then balances the intake against it.
Total NFA = Attic floor area ÷ 150 (or ÷ 300)
Exhaust NFA = Total NFA ÷ 2
Exhaust vents = ⌈ Exhaust NFA ÷ NFA per exhaust vent ⌉ · Ridge vent length = Exhaust NFA ÷ NFA per foot
Intake vents = ⌈ Exhaust vents × NFA per exhaust vent ÷ NFA per intake vent ⌉
⌈ ⌉ means rounding up to the next whole vent. Areas are converted with 1 sq ft = 144 sq in (1 m² = 10,000 cm²).
The ratio comes from your code, not from this page
The International Residential Code, section R806.2, sets the minimum net free ventilating area at 1/150 of the vented space. It allows 1/300 in two cases: in climate zones 6, 7 and 8 when a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling, or when not less than 40 % and not more than 50 % of the required area is in vents in the upper part of the attic – no more than 3 ft below the ridge – with the rest in eave or cornice vents. The calculator shows both ratios as a reference. Local codes amend these rules, roof and insulation makers have their own requirements, and some attics are built unvented, so check with your building department.
Net free area, not vent size
A vent’s outside size says little about how much air it passes. Lomanco’s C816 soffit vent is 8 × 16 in overall with a 7 × 15 in opening, yet it is rated at 65 sq in of NFA because the louvers and screen block part of the opening. Ridge vents are rated per foot: Lomanco lists 18 sq in of NFA per foot for its LPR and VUR vents, so a 10 ft VUR-10 piece gives 180 sq in. The 750 roof vent gives 50 sq in. Always divide by the NFA the maker prints for your vent.
Balancing intake and exhaust
Lomanco divides the required area equally between exhaust and intake and then sizes the intake to the exhaust actually installed: intake NFA should meet or slightly exceed it. The calculator does the same. Because whole vents are rounded up, the installed exhaust is usually a little more than half the total, and the intake is matched to that – so the installed total never falls below the requirement.
Ridge length
A ridge vent needs ridge to sit on. If you enter the ridge length available, the calculator warns you when the vent length needed is longer. Hips, end cut-backs and chimneys shorten the usable ridge. When the ridge is too short, the vent maker’s instructions tell you which other exhaust vents can be combined with it.
Worked example in words
A 1,200 sq ft attic at 1/150 needs 8 sq ft of net free area – 1,152 sq in. Half of it, 576 sq in, goes to the ridge. With 180 sq in per 10 ft piece that is 3.2 pieces, so you buy 4 pieces (32 ft of vent is needed) and install 720 sq in of exhaust. Matching it with 65 sq in soffit vents takes 720 ÷ 65 = 11.1, so 12 soffit vents giving 780 sq in.
Worked example
Inputs
- Attic floor area
- 1200 sq ft
- Ventilation ratio
- 1/150 – 1 sq ft of NFA per 150 sq ft
- Exhaust vent
- Lomanco VUR-10 ridge vent – 10 ft piece, 180 sq in (18 sq in per ft)
- Intake vent
- Lomanco C816 soffit vent, 8 × 16 in – 65 sq in each
Result
- Total net free area
- 1,152 sq in (8 sq ft)
- Exhaust NFA required (half)
- 576 sq in (4 sq ft)
- Exhaust vents or ridge vent pieces
- 4
- Ridge vent length needed
- 32 ft
- Exhaust NFA installed
- 720 sq in (5 sq ft)
- Intake vents
- 12
- Intake NFA installed
- 780 sq in (5.42 sq ft)
Common mistakes
- Using the roof area instead of the attic floor area. The ratio is applied to the area of the vented space, so a steep roof does not need more NFA than a shallow one over the same ceiling.
- Counting vents by their outside size. A Lomanco 8 × 16 in soffit vent has a 7 × 15 in opening but is rated at 65 sq in of net free area, far less than 128 sq in.
- Using 1/300 without meeting the conditions. The IRC allows the smaller ratio only with a ceiling vapor retarder in climate zones 6–8 or with 40–50 % of the area in upper vents near the ridge.
- Installing more exhaust than intake. Lomanco says intake NFA should meet or slightly exceed the installed exhaust NFA; too little intake starves the exhaust vents.
- Forgetting to round up. Lomanco’s own example needs 9.6 roof vents and installs 10.
Assumptions and limits
- Total NFA = attic floor area ÷ the ratio you choose (150 or 300). The result is shown in square inches (cm² in metric); 1 sq ft = 144 sq in.
- Half the total goes to exhaust, as in Lomanco’s method; the intake is then sized to at least the exhaust NFA you actually install, so the total installed is never below the requirement.
- Vents are counted from each maker’s NFA rating and rounded up to whole vents or whole ridge vent pieces. Ridge vent length is the exact length that gives the required exhaust NFA.
- One exhaust type and one intake type. Mixing ridge vents with gable or powered vents, bathroom fan ducts and vent chutes at the insulation are not modelled – follow the vent maker’s instructions.
Frequently asked questions
How much attic ventilation do I need?
The International Residential Code (R806.2) sets the minimum net free ventilating area at 1/150 of the vented attic area, or 1/300 under specific conditions. For a 1,200 sq ft attic at 1/150 that is 8 sq ft, or 1,152 sq in, split between intake and exhaust. Your local code has the final say.
What is net free area (NFA)?
Lomanco defines it as the actual open area through which air can pass after the vent’s louvers, screens and other parts are accounted for. It is measured in square inches and is smaller than the vent’s outside dimensions, so always use the NFA printed by the maker.
How many feet of ridge vent do I need?
Divide the exhaust NFA by the ridge vent’s NFA per foot. Lomanco’s LPR and VUR ridge vents have 18 sq in per foot, so 576 sq in of exhaust needs 32 ft of vent – four 10 ft pieces.
How many soffit vents do I need?
Divide the exhaust NFA you install by the NFA of one soffit vent and round up. With four Lomanco ridge vent pieces (720 sq in) and C816 soffit vents at 65 sq in each, that is 720 ÷ 65 = 11.1, so 12 vents.
Should intake and exhaust be equal?
Lomanco divides the required area equally and says intake should meet or slightly exceed the installed exhaust. When the IRC’s 1/300 exception relies on vent placement, 40–50 % of the area must be in upper vents and the rest at the eaves or cornice.
Sources
- Seattle Residential Code 2015, Chapter 8 – Roof-Ceiling Construction (2015 IRC with amendments) — City of Seattle, Department of Construction and Inspections. R806.2: "The minimum net free ventilating area shall be 1/150 of the area of the vented space"; exception: 1/300 where, in Climate Zones 6–8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling, or "not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion", not more than 3 ft below the ridge, the balance by eave or cornice vents.
- How Many Attic Vents Do I Need? — Lomanco. Divide the attic floor area by the ratio (1/150, or 1/300 where permitted), × 144 for square inches of NFA, split between intake and exhaust; "Always round up to the next whole vent"; intake NFA "should meet or slightly exceed the installed exhaust NFA"; example 2,000 sq ft at 1/300 → 960 sq in, ten 750 vents (50 sq in each) = 500, fourteen Deck-Air DA-4 (36 sq in each) = 504.
- Ridge Vents – model information — Lomanco. LPR and VUR aluminum ridge vents: "18 square inches of NFA per foot"; model table VUR-10: length 10 ft, NFA 180 sq in (LPR-8: 8 ft, 144 sq in).
- Static Roof Vents – model information — Lomanco. Model table: 750 slant-back roof vent, 16 × 20.313 × 5 in, NFA 50 sq in.
- Intake Vents – model information — Lomanco. Soffit vent table: C816, 8 × 16 in overall, 7 × 15 in opening, NFA 65 sq in (C416 25, C616 37); "The intake system must provide as much as or more than the exhaust system in Net Free Area."
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026