Attic Ventilation Calculator: NFA and Intake Balance
Attic Ventilation Calculator: NFA and Balance
Net free area required for an attic, split between intake at the soffit and exhaust at the ridge, then converted into an actual count of vents from the NFA printed on the product you are buying.
Net free area is the open area of the vent after its louvres and insect screen, which is always less than the hole it covers, and it is printed on the packaging. Intake and exhaust must be roughly equal: exhaust without matching intake pulls conditioned air up out of the house instead of drawing outside air through the attic, which is why adding ridge vent alone can make a moisture problem worse rather than better.
How to use this tool
Enter the attic floor area and choose the ventilation ratio. The tool returns the total net free area required, splits it between intake at the soffit and exhaust at the ridge, and converts each into a real quantity: feet of ridge vent and a count of soffit vents, using the net free area printed on the products you are actually buying.
Net free area is not the size of the hole
A vent is a hole covered by louvres and insect screen, and both obstruct airflow. Net free area is what remains open after those obstructions, and it is always meaningfully less than the opening it sits in. It is printed on the packaging in square inches, and it is the only number worth calculating with. Sizing ventilation by counting vents, or by the size of the holes cut, overstates the actual ventilation every time.
Why balance matters more than total area
The default requirement is 1 square foot of net free area for every 150 square feet of attic floor. The more generous 1/300 ratio is permitted only where the ventilation is balanced, commonly with 40 to 60 percent of it in the upper portion, and where the other conditions in your adopted code are satisfied.
The balance condition is doing real work, and it is worth understanding rather than merely obeying. Attic ventilation is driven by air rising: warm air leaves at the ridge and outside air is drawn in at the soffit to replace it. If exhaust is generous and intake is starved, the air still has to come from somewhere, and the easiest path is upward through ceiling penetrations from the conditioned house below. The attic then ventilates itself with heated, humidified indoor air, which is precisely the moisture you were trying to remove. This is why adding ridge vent to a house with blocked soffits can make condensation and ice damming worse rather than better, and why the intake number on this tool matters at least as much as the total.
A worked check you can repeat on paper
A 1500 square foot attic at the 1/300 ratio needs 5 square feet of net free area, which is 720 square inches. Split evenly, that is 360 square inches at the ridge and 360 at the soffits. Ridge vent rated at 18 square inches per foot needs 20 feet of it; soffit vents rated at 56 square inches each need 7. At the 1/150 ratio the same attic needs 1440 square inches, exactly double, which is the arithmetic behind why the balance condition is worth meeting.
Mistakes that catch people out
Claiming 1/300 without balance. The generous ratio is conditional. Fix: the tool flags a split outside 40 to 60 percent instead of silently allowing it.
Counting the opening rather than the net free area. Fix: enter the NFA printed on the product.
Exhaust without intake. The failure mode described above, and the most damaging one. Fix: the intake line is given equal prominence in the output.
Mixing exhaust types. Ridge vent combined with powered or box vents can short circuit, pulling air from the nearest opening instead of from the soffits. Fix: choose one exhaust strategy and size it here.
How this was checked
The ratio arithmetic was verified on the worked example: 1500 square feet at 1/150 gives exactly 10 square feet, or 1440 square inches, and the 1/300 case gives exactly half of it. The even split was confirmed at 720 and 720, the balance flag was confirmed to accept 50 percent and reject 70, and the vent counts were checked at the rounding boundary, where 1440 square inches at 60 each gives exactly 24 and 1441 gives 25.
Related Calculators and Reference
See also the R Value to RSI Converter for the insulation side of an attic, where Canadian codes quote RSI and the batts are stamped R.
Roof and attic areas come from the Area of 2D Shapes Calculator, and unit changes are the Area Converter. Insulating the attic the ventilation protects pairs with the Paint Coverage Calculator for the finishing work below it.