Enter attic floor area — not roof area. Pick ratio, exhaust, intake. Get required net free area, balanced split, and exact vent count.
Optional: override net free area per unit
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How this calculator works
Attic ventilation is sized to the ventilated space — the floor area under the roof, not the sloped roof surface. IRC R806.2 requires a minimum net free ventilation area of 1:150 of the attic floor area, unless a Class I or II vapor retarder is installed and the intake/exhaust split falls within a specific range, in which case 1:300 is permitted.1 This calculator uses the exact code formula and then derives intake and exhaust targets from it.
Net free area (NFA) is what matters, not the physical opening size. A 5″ ridge vent might measure 1¼″ of open slot per foot, but it is rated at about 18 sq in of NFA per linear foot because internal baffles block part of the opening. Using gross area instead of NFA is the single biggest ventilation calculation error.3
Balanced ventilation moves air in at the soffit and out at the ridge. Without intake, ridge vents pull air from any path they can find — including conditioned space below.
Intake vs exhaust — the balance rule that matters most
Balanced intake and exhaust matters more than total NFA. If the exhaust is oversized and the intake is undersized, the ridge vent pulls air from any leak path it can find — recessed lights, attic hatch gaps, HVAC penetrations. That pulls conditioned air out of the house, raises HVAC bills, and can pull moisture into the attic in cooling climates.4 It also makes the exhaust vent appear to work while actually under-ventilating the attic.
Same total net free area, different performance. The unbalanced system does not ventilate the attic — it ventilates the house.
Net free area by vent type
These are the NFA values the calculator uses as defaults. Every one of them is a net free area rating, not the physical opening. Override any of them if your product label states different numbers.
Field note from the estimating desk: the most expensive attic ventilation mistake is adding a ridge vent without checking the soffit. A homeowner installs a ridge vent on an older house with painted-shut soffits or no soffit vents at all, and within two years the attic sheathing shows moisture staining. The ridge vent is not the problem — it is that the ridge vent has no intake air to pull. Budget for new soffit vents at the same time you add the ridge vent, or skip the ridge vent entirely. Never add exhaust without confirming intake.
Frequently asked questions
How much attic ventilation do I need?
Divide your attic floor area by 150 to get the required net free area in square feet, then multiply by 144 for square inches. A 1,500 sq ft attic needs 1,440 sq in of NFA (10 sq ft). Split it roughly 50/50 between intake and exhaust unless you are using the 1:300 exception, in which case 50–80% of the total must be exhaust and 20–50% intake.
What is net free area?
Net free area (NFA) is the open area of a vent after internal baffles, screens, and louvers are subtracted. A ridge vent that physically measures 1¼″ of slot per foot might only deliver 18 sq in of NFA per linear foot. Ventilation must always be calculated in NFA, not gross opening size — using gross area overstates the ventilation by 40–60%.3
What is the 1:150 vs 1:300 rule?
The base IRC requirement is 1:150 — one square foot of net free area per 150 square feet of attic floor. A 1:300 ratio is permitted only as an exception: a Class I or II vapor retarder must be installed on the warm-in-winter side of the ceiling, and the intake/exhaust split must fall between 50–80% exhaust and 20–50% intake.1 If either condition is not met, use 1:150.
Why does intake matter as much as exhaust?
Exhaust vents can only remove air that is replaced from somewhere. If intake is undersized, the exhaust pulls air from conditioned space through can lights, attic hatch gaps, and HVAC penetrations — raising HVAC costs and pulling moisture into the attic in cooling climates.4 A house with ridge vents and no soffit intake is ventilating the house, not the attic.
How many soffit vents do I need?
For a 1,500 sq ft attic at 1:150, the intake target is about 720 sq in. Continuous soffit strip vent at 9 sq in / LF needs about 80 linear feet of strip. Individual 8×16 soffit panels at 50 sq in each need about 15 panels. Distribute evenly on both sides of the roof for even airflow.
Can I have too much ventilation?
In cold climates, yes — excessive ventilation without a good air barrier and insulation contact can cause ice dams or cold attic conditions. In most of the country, the practical failure is too little intake, not too much total. The exception in cold climates is an over-ventilated attic with a poor air barrier, which loses heat too quickly and can drive ice damming.
How long should a ridge vent be?
Almost always the full length of the ridge minus 6 inches at each gable end. At 18 sq in of NFA per linear foot, a 40 ft ridge delivers about 720 sq in of exhaust — more than enough for a 1,500 sq ft attic at 1:150, but only if intake is sized to match.
Do I need ventilation if I have a spray foam attic?
If the attic is conditioned (spray foam on the underside of the roof deck, no thermal boundary at the ceiling), the attic is inside the building envelope and does not need ventilation — this calculator does not apply. If the spray foam is on the attic floor only, the attic is still unconditioned and needs ventilation sized to code.
What is the difference between gable vents and ridge vents?
Both are exhaust vents. Gable vents are on the end walls and only ventilate near that end of the attic. Ridge vents run continuously along the peak and provide even exhaust along the entire length of the roof. Ridge vents plus soffit intake is the preferred system because it produces even airflow; gable vents work but leave dead spots at the middle of the ridge.
Standards and sources
International Code Council — IRC R806.2 Minimum Vent Area. 1:150 base ratio and the 1:300 exception with vapor retarder and 50–80% upper-vent condition. codes.iccsafe.org
National Roofing Contractors Association — Ventilation Guidelines. Balanced intake/exhaust and the 1:150 / 1:300 clarification for residential attic ventilation. nrca.net
Asphalt Roofing Manufacturers Association — Attic Ventilation: Why Net Free Area Matters. NFA vs gross opening size, and the 40–60% overstatement of gross area. asphaltroofing.org
US Department of Energy — Attic Ventilation and Moisture Control. Consequences of unbalanced ventilation and conditioned-space air pull in cooling climates. energy.gov
Home Ventilating Institute — Certified Ventilation Products Directory. Manufacturer-rated NFA for ridge, soffit, box, turbine, and powered vents. hvi.org
BM
Builders Measures estimating team
Reviewed against IRC R806.2 and NRCA ventilation guidance. NFA values are industry-standard defaults — always confirm the rated NFA on the product label for the specific vent you are installing. Last reviewed October 2026.
Estimates only. Confirm the rated net free area on your vent product label and verify local code requirements before installing. Last reviewed October 2026.