Attic ventilation calculator (IRC R806)

Attic floor area

This is the floor, not the roof. R806.2 sizes ventilation off the area of the vented space, which is the ceiling plane below the attic. Using the sloped roof area instead over-ventilates by the slope factor.

Code rule

Exhaust product (high on the roof)

Intake product (low, at the eaves)

Share provided as exhaust

Ridge length (optional)

Existing exhaust NFA to credit

Existing intake NFA to credit

Credit them separately. Intake and exhaust do not substitute for each other, so the two sides are sized against their own share. An attic with plenty of soffit intake and no exhaust needs all of its new area up high, not half of each.

Required net free area — 1/150 — the base rule

Required NFA

1,440sq in

10 sq ft of net free area

Attic floor area

1,500sq ft

Divided by 150

Still to install

1,440sq in

Nothing credited as existing

Exhaust — still to install (50% share)

720sq in

720 sq in required, none existing

Intake — still to install (50% share)

720sq in

720 sq in required, none existing

Finished system: 720 sq in of exhaust against 720 sq in of intake. Balanced — intake is at least equal to exhaust, which is what you want.

Product to install

Vent product to install
Position	Product	NFA needed	Install
Exhaust	GAF Cobra RidgeRunner ridge vent (12.5 sq in/ft) 12.5 sq in per lineal ft	720sq in	58lineal ft
Intake	Soffit panel vent, 8 x 16 in aluminium (56 sq in each) 56 sq in per piece	720sq in	13pieces

Quantities are rounded up to whole feet and whole pieces, so each side covers at least the net free area it needs.

Read this before you order

Net free area is a property of the specific product and is printed on its label. Every figure here names a real product and cites where it came from, but your product governs, not ours.

Rule applied: 1/150 — the base rule. IRC R806.2 base requirement. Always permitted; use it when the exception's conditions are not met or not certain.

Keep this one

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The formula

required NFA (sq ft) = attic floor area / divisor divisor = 150 or 300 required NFA (sq in) = required NFA (sq ft) × 144 required exhaust = required NFA × exhaust share required intake = required NFA × (1 − exhaust share) exhaust to install = max(0, required exhaust − existing exhaust NFA) intake to install = max(0, required intake − existing intake NFA) quantity = ceil( that side's NFA to install / product NFA per unit ) ridge check = exhaust run (lineal ft) ≤ ridge length, when a ridge length is given

IRC R806.2 sets the minimum net free ventilating area at 1/150 of the area of the vented space. The vented space is measured on the attic floor — the ceiling plane — not on the sloped roof above it. An exception permits 1/300, with conditions this calculator raises where the product you picked is the one likely to breach them.

Intake and exhaust are sized separately, and existing ventilation is credited against its own side. They are not interchangeable: an attic with 400 sq in of soffit intake and no exhaust at all does not need half of each, it needs exhaust. Crediting one pooled number and splitting the remainder is the arithmetic that leaves a roof short of exhaust and over-supplied with intake.

What the 1/300 exception asks for, and why net free area is not the size of the hole

The exception applies, in the adopted 2021 text, only if both of two conditions are met: in Climate Zones 6 to 8 a Class I or II vapour retarder on the warm-in-winter side of the ceiling, and 40 to 50% of the required ventilating area provided by ventilators in the upper portion of the attic. That second condition carries two more sentences that are easy to miss: the upper ventilators must sit not more than 3 feet below the ridge measured vertically, and the balance of the ventilation must be located in the bottom one-third of the attic space.

Net free area is not the size of the hole. It is the open area left after the louvres, the insect screen and the baffle, and it is a property of the specific product, printed on its label. Every figure here names a real product and cites where the figure came from, because the spread is wide enough to change the order: real continuous ridge vents run from 12.5 sq in per foot (GAF Cobra RidgeRunner) to 18 (Air Vent ShingleVent II, per ICC-ES ESR-2071), a 44% swing. The default is the most conservative of them. Each side’s area is then divided by the product’s net free area per foot or per piece and rounded up, because you cannot buy two thirds of a soffit panel.

Worked example: a 1,500 sq ft attic on a 40 ft ridge

Attic floor area: 1,500 sq ft
Rule: 1/150 (base)
Exhaust: GAF Cobra RidgeRunner, 12.5 sq in per ft
Intake: 8 × 16 in aluminium soffit panel, 56 sq in each
Split: 50 / 50
Ridge length: 40 ft (optional input)

Required area. 1,500 / 150 = 10.00 sq ft of net free area.
In square inches. 10.00 × 144 = 1,440 sq in.
Required exhaust. 1,440 × 50% = 720 sq in. Nothing existing to credit, so 720 sq in to install.
Ridge vent. 720 / 12.5 sq in per ft = 57.6, so 58 lineal ft of Cobra RidgeRunner.
Required intake. 1,440 × 50% = 720 sq in, again with nothing to credit.
Soffit panels. 720 / 56 sq in each = 12.86, so 13 panels.
Check the run against the ridge. 58 lineal ft will not fit on a 40 ft ridge. The full ridge carries 40 × 12.5 = 500 sq in, leaving 720 − 500 = 220 sq in to find elsewhere: 220 / 50 sq in per box vent = 4.4, so 5 static vents set within 3 ft of the ridge — or pick a ridge vent with a higher net free area per foot.
The same attic under the 1/300 exception. 1,500 / 300 = 5.00 sq ft = 720 sq in required. The two sides are 360 sq in each: 360 / 12.5 = 28.8, so 29 lineal ft of RidgeRunner, and 360 / 56 = 6.43, so 7 panels. 29 ft fits the 40 ft ridge.
Why the product matters more than the rule. That same 360 sq in on Air Vent ShingleVent II at 18 sq in per ft is 360 / 18 = 20 lineal ft exactly. A calculator that assumes 18 and hands you 20 ft leaves a RidgeRunner job 9 ft short of the 29 it needs — about a third of the exhaust missing, and found after the roof is on. The default here is 12.5 for that reason.
What the exception costs you in paperwork. Halving the vent area is permitted only if BOTH conditions are met: 40–50% of the required area in upper ventilators sitting no more than 3 ft below the ridge, with the balance in the bottom one-third of the attic, AND in Climate Zones 6 to 8 a Class I or II vapour retarder on the warm side of the ceiling. Earlier editions asked for only one or more of the conditions, so the adopted edition decides it, not this page.

Under 1/150: 1,440 sq in of net free area — 720 sq in a side. That is 58 lineal ft of Cobra RidgeRunner, which does not fit a 40 ft ridge, so 40 ft of ridge vent plus 5 box vents, and 13 soffit panels. Under the 1/300 exception: 720 sq in — 29 lineal ft of ridge vent and 7 panels. Either way you leave the supplier with what the roof needs, and the inspector has nothing to write up.

Figures from IRC R806.2, Minimum vent area — adopted 2021 text; ICC-ES ESR-2071 — Air Vent ShingleVent II, 18 sq in/ft; ICC-ES ESR-2484 — DCI SmartVent roof-edge intake, 8.9 sq in/ft; GAF — Cobra RidgeRunner, 12.5 sq in/ft.

Questions

Do I measure the attic floor or the roof?

The attic floor. R806.2 sizes ventilation off the area of the vented space, which is the ceiling plane under the attic. The sloped roof above it is larger by the slope factor, so using the roof area instead would over-ventilate a 6/12 roof by about 12% and a 12/12 by 41%. The only time the two coincide is a flat ceiling under a flat roof.

Can I use the 1/300 rule?

Sometimes, and it is not this calculator's decision. The 1/300 figure is an exception rather than the rule, and the adopted 2021 IRC R806.2 permits it only where BOTH conditions are met: in Climate Zones 6 to 8 a Class I or II vapour retarder on the warm-in-winter side of the ceiling, and 40 to 50% of the required ventilating area in upper ventilators. That second condition also requires the upper ventilators to sit no more than 3 feet below the ridge measured vertically, and the balance of the ventilation to be located in the bottom one-third of the attic space. Earlier editions were looser, not stricter — the 2012 ICC code change proposal form quotes the then-current text as permitting 1/300 where 'one or more of the following conditions are met', so a builder could satisfy either. Your jurisdiction's adopted edition governs, so check it there and keep 1/150 if you are not certain.

Which ridge vent figure should I use?

The one on your product's label, and the spread is bigger than most people expect. Air Vent's ShingleVent II is rated at 18 sq in of net free area per lineal foot in ICC-ES ESR-2071, which is about the top of the market. GAF's Cobra family — what a GAF-system roof usually gets — is materially lower: RidgeRunner is 12.5, the rolled Cobra Exhaust Vent is 16.9 hand-nailed and 14.1 nail-gunned, and Cobra Rigid Vent 3 is 18. A tool that quietly assumes 18 will tell you 20 lineal ft where a RidgeRunner job needs 29. This calculator defaults to the lowest of them and makes you pick up if your product is more generous, because being long on ridge vent costs a few dollars and being short fails an inspection.

I already have soffit vents. Can I just subtract them from the total?

No, and this is the mistake worth checking any calculator for. Intake and exhaust do not substitute for each other, so existing ventilation has to be credited against the side it is actually on. Take an attic needing 720 sq in with 400 sq in of soffit intake and no exhaust at all. Subtracting 400 from 720 and splitting the remainder gives 160 sq in of exhaust and 160 of intake — when what the roof needs is about 360 sq in of exhaust and no more intake. That is why there are two existing-NFA fields here rather than one, and why each side is sized against its own share.

What if the ridge is shorter than the vent run?

Then you cannot have the run, and the calculator says so instead of letting you order it. Give it the ridge length as the optional input and it checks the required continuous run against what exists. When the run is longer, it tells you how much net free area the full ridge can carry, how much is left over, and roughly how many static vents at 50 sq in each would make up the difference — set within 3 feet of the ridge, so they still count as upper ventilators under the exception. Picking a ridge vent with a higher net free area per foot is the other way out. What you should not do is fit 58 feet of vent on a 40 foot ridge and call the attic ventilated.

Does a roof-edge or drip-edge intake vent count?

It is a real product, it is often the only option on an eave with no soffit, and it is a live argument with inspectors. R806.2 requires that the balance of the required ventilation be located in the bottom one-third of the attic space, and a roof-edge intake sits on the roof deck above the eave — so it is neither an eave or cornice vent nor inside the bottom third of the attic. Inspectors do not agree on whether that satisfies the sentence. This calculator offers the product and sizes it from ICC-ES ESR-2484 at 8.9 sq in per lineal foot, but it warns you when you choose it. Get it accepted before you buy a roll of it, especially if you are relying on the 1/300 exception.

What is net free area, and why is it smaller than the vent?

Net free area is the open area air can actually pass through once the louvres, the insect screen and any baffle have taken their share. A vent with a 10 × 10 in opening does not offer 100 sq in of NFA. It is a property of the specific product and the manufacturer prints it on the label or the data sheet. Two 8 × 16 in soffit panels make the point: the aluminium one is 56 sq in and the plastic one is 45.5, a fifth less open at the same size. Every figure in the picker here names its product and cites its source for that reason.

Why should intake and exhaust be equal?

Because an attic ventilates by moving air from low to high, and the slower half sets the rate. If exhaust is larger than intake, the shortfall gets made up from the easiest opening available, which is usually the house itself through ceiling penetrations — pulling conditioned air into the attic. In wind-driven rain or snow it can also draw water back in through the exhaust vents. More intake than exhaust is the safe direction to be wrong in, and the calculator flags the finished system, not just the split you asked for: credit 800 sq in of existing exhaust against a 360 sq in requirement and it will tell you to raise intake to meet it, because you cannot un-install a vent.

Can I mix ridge vent with box vents?

On the same roof plane it tends to short-circuit. A ridge vent and box vents will often exchange air with each other rather than with the eaves, so the lower part of the attic stops ventilating, and power vents mixed with a ridge vent do the same thing more forcefully. The exception is the case above — a ridge too short to carry the required run — where the make-up vents are there because the ridge ran out, not as a second system. Pick one exhaust type per attic space and give it intake low down, which is why this calculator takes one exhaust product and one intake product.

What about a cathedral ceiling or an unvented assembly?

Neither is what this calculator sizes. A vented cathedral ceiling needs a continuous clear path in every rafter bay, so it is sized per bay rather than off a floor area. An unvented assembly under R806.5 is a different approach entirely, with its own insulation and vapour-retarder requirements and no ventilating area at all. This page assumes a conventional vented attic over a flat ceiling.