Attic Ventilation Calculator
Size attic ventilation by Net Free Area using the IRC 1/150 and 1/300 rules, balance intake and exhaust, and convert to linear feet of ridge and soffit vent.
The footprint being ventilated.
Balanced intake/exhaust + high exhaust or vapor retarder. IRC R806.2: 1/300 needs a vapor retarder, or 40–50% of the venting as high exhaust with balanced intake.
Intake should equal or slightly exceed exhaust. 60/40 intake-favor is good practice.
Pick one exhaust strategy — don't mix types.
Continuous soffit vent is ~9 sq in/ft. Check your product.
Required ventilation
5 sq ft total NFA
360 sq in of soffit vent
360 sq in of exhaust
- Ratio rule
- 1/300
- Intake / exhaust split
- 50 / 50
Balanced ventilation matters: intake should equal or slightly exceed exhaust. Too much exhaust with too little intake pulls conditioned air from the house.
Don’t mix exhaust types (ridge + box + gable) — they short-circuit each other. Pick one exhaust strategy.
NFA values vary by product. Use the manufacturer’s stated NFA for your specific vent.
The 1/300 ratio requires balanced intake/exhaust with the exhaust high on the roof, or a vapor retarder; otherwise use 1/150 (IRC R806.2). Verify against current local codes.
This sizes ventilation area only — also address air sealing, insulation, and bath-fan venting.
How this calculator works
Attic ventilation keeps a roof assembly dry and at an even temperature — flushing out summer heat and winter moisture before they rot sheathing, ruin insulation, or feed ice dams. This calculator sizes the ventilation by Net Free Area (NFA), splits it into balanced intake and exhaust, and converts the result into the linear feet or unit counts you actually order.
Step 1: Required Net Free Area
The code rule (IRC R806.2) is a ratio of attic floor area. The baseline is 1/150 — one square foot of NFA per 150 square feet of attic. You may use the more forgiving 1/300 ratio when a vapor retarder is present, or when 40–50% of the ventilation is high exhaust balanced by low intake. A 1,500 sq ft attic needs 10 sq ft of NFA at 1/150, or 5 sq ft at 1/300.
Step 2: Balance intake and exhaust
Total NFA is split between intake (low, at the soffits/eaves) and exhaust (high, at the ridge). Intake should equal or slightly exceed exhaust — a 50/50 split is the floor, and 60/40 in favor of intake is preferred. Exhaust vents can only move the air that intake feeds them, so an exhaust-heavy attic ends up pulling conditioned air out of the house instead.
Step 3: Convert to product
NFA is converted to linear feet or counts using each product's rating: continuous ridge vent runs about 18 sq in per foot, continuous soffit vent about 9 sq in per foot, and box vents roughly 50 sq in each. The calculator rounds runs and counts up, because a partial vent run does not perform. These are typical values — always use the NFA printed on your specific product.
One exhaust strategy, please
Do not mix exhaust types. A ridge vent combined with box or gable vents short-circuits: the highest vent draws air from the lower exhaust openings rather than from the soffit intake, leaving part of the roof unventilated and inviting weather inside. Pick a single exhaust path and pair it with enough intake. Verify everything against the code edition your jurisdiction has adopted.
Frequently asked questions
- What is Net Free Area (NFA)?
- Net Free Area is the actual open area of a vent through which air can move, measured in square inches, after subtracting the louvers, screens, and baffles that block part of the opening. It is always smaller than the vent's overall size. Ventilation requirements are written in terms of NFA, and every reputable vent product publishes its NFA rating.
- What is the 1/150 vs 1/300 rule?
- Per IRC R806.2, attic ventilation must provide at least 1 square foot of net free area for every 150 square feet of attic floor (1/150). That requirement drops to 1/300 when a Class I or II vapor retarder is installed on the warm side of the ceiling, OR when at least 40% and no more than 50% of the ventilation is provided by exhaust located in the upper portion of the attic with the balance as low intake — in other words, balanced ventilation with the exhaust high on the roof. If you can't meet those conditions, use 1/150.
- Should intake or exhaust be larger?
- Intake should always equal or slightly exceed exhaust. A 50/50 split is the minimum, and a 60/40 intake-favor is good practice. The reason: exhaust vents only work if there is enough intake feeding them. If exhaust outpaces intake, the system pulls makeup air from the easiest available source — often conditioned air from inside the house through ceiling gaps.
- Can I mix ridge vents and box vents?
- No. Mixing exhaust types — a ridge vent plus box vents, or a ridge vent plus gable vents — short-circuits the system. The ridge vent, being highest, can pull air in through the lower exhaust vents instead of from the soffit intake, so part of the roof gets no real airflow and weather can be drawn in. Pick one exhaust strategy and pair it with adequate intake.
- How much ridge vent do I need for 1,500 sq ft of attic?
- At the 1/300 ratio, 1,500 sq ft needs 5 sq ft of total NFA (720 sq in). Split 50/50, that's 360 sq in each for intake and exhaust. Continuous ridge vent at ~18 sq in per foot needs 360 ÷ 18 = 20 linear feet of ridge; continuous soffit vent at ~9 sq in per foot needs 360 ÷ 9 = 40 linear feet of soffit. At 1/150 you'd double both.
- What happens with too much exhaust and not enough intake?
- An exhaust-heavy, intake-starved attic depressurizes and pulls its makeup air from inside the house — wasting conditioned air, drawing moisture up into the attic, and in winter feeding warm air that melts snow unevenly and contributes to ice dams. Adding more exhaust does not fix poor ventilation; balancing it with adequate intake does.