Attic Ventilation Calculator (NFA, Soffit Intake & Exhaust Balance)
Size attic ventilation to the 1/150 or 1/300 rule: net free area required for the attic floor area, split 50/50 or 60/40 between low intake (soffit) and high exhaust (ridge, box, turbine or gable vents), how many feet of soffit and ridge vent or how many vents that takes with your products’ own NFA ratings, whether your current vents are balanced, rafter baffles and a powered-fan CFM alternative.
How it works
NFA (ft²) = attic floor area ÷ 150, or ÷ 300 when 40–50% of it is high and the rest low (IRC R806.2) · in² = ft² × 144 · intake = NFA × 50% (or 60%), exhaust = the rest · feet of vent = in² ÷ product NFA per ft; vents = in² ÷ NFA per vent, rounded up · typical NFA: ridge vent 18 in²/ft, soffit strip 9 in²/ft, 16×8″ vent 56 in², box vent 50 in², 12″ turbine 100 in² · powered fan ≈ 0.7 CFM per ft² (+15% for a dark roof) with 1 ft² of intake per 300 CFM
An attic breathes by convection: cool air enters low at the soffits and warm, moist air leaves high at the ridge. The code sets the minimum opening as a fraction of the attic floor area: one square foot per 150, or per 300 when 40–50% of the vents are high and the rest low (with a vapour retarder in cold climates). The number that matters is net free area, the open area left after louvres and screens, so the calculator divides by each product’s rating. You can use the typical ratings or enter the ones printed on your vents. Intake should be at least as large as exhaust, which is why a 60/40 split in favour of intake is offered. The result tells you how much soffit and ridge that takes, whether it fits the eaves and ridge you have, and how your current vents compare. Balance is the usual failure: plenty of ridge vent over blocked soffits makes the ridge inhale instead of exhale, drawing conditioned air and moisture out of the house.
Worked example
A 50 × 30 ft attic (1,500 ft²) under the 1/300 rule needs 5 ft², or 720 in², of net free area: 360 in² of intake and 360 in² of exhaust. That is 40 ft of continuous soffit strip vent (or 7 rectangular 16×8″ vents) and 20 ft of ridge vent, well within the 100 ft of eave and 50 ft of ridge available. With a 60/40 split the intake rises to 432 in² (48 ft of strip vent) and the exhaust falls to 288 in² (16 ft of ridge vent). A powered fan alternative would be about 1,050 CFM with at least 504 in² of intake.
Sources: International Residential Code R806.2 — minimum attic ventilation area
Frequently asked questions
Can I have too much ventilation?
Too much exhaust relative to intake, yes — it depressurises the attic and pulls house air. Extra intake is harmless. Ridge vents along the whole ridge are fine only if the soffits supply at least as much NFA.
Should I add a powered attic fan?
Usually not to a house with a working ridge-and-soffit system — the fan short-circuits it and can suck conditioned air (and combustion gases) up through ceiling leaks. Fans suit hip roofs with little ridge, and only with ample soffit intake and no ridge vent.
What is net free area (NFA)?
The open area of a vent once louvres, screens and baffles are subtracted, in square inches. It is usually 40–60% less than the hole, so always use the figure printed on the product. Enter it under More options and the calculator counts with your product instead of a typical one.
Should intake and exhaust be equal?
At least equal, and many vent makers prefer slightly more intake. The code’s 1/300 rule asks for 40–50% of the vent area high and the rest low; choose the 60/40 split to size for extra intake.
How do I check my existing attic ventilation?
Count your vents and add up their NFA ratings: intake (soffit, edge vents) and exhaust (ridge, box vents, turbines). Enter both totals and the calculator compares them with the requirement and tells you whether you are short, exhaust-heavy or balanced.
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Home-exterior rules of thumb from manufacturers, building-science guides and code commentary. Local codes, climate and site conditions govern — confirm structural, electrical and drainage work with a qualified contractor or your building department.