Cutaway diagram of a ventilated attic showing cool blue intake air entering through soffit vents and warm red exhaust air leaving through the ridge vent.

Ventilation calculator

How much attic ventilation does your roof actually need?

This computes your exact net free area requirement — intake, exhaust, and vent length — instead of handing you a rule of thumb.

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Content reviewed and approved by Jose Puente, Civil Engineer and owner of Atrium Roofing since 2016.

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Net free area is the number that matters

Not how many vents you have. How much actual open area they add up to.

Net free area, or NFA, is the real open area of a vent — what is left after the louvers, baffles, and insect screen are subtracted from the outside dimensions. A vent that looks large can pass very little air. Code is written in net free area for exactly that reason, and so is every ridge vent spec sheet.

The minimum is 1 sq ft of net free ventilation area for every 300 sq ft of attic floor when a vapor retarder is in place, or 1:150 without one. That total gets split roughly 50/50 between intake at the soffits and exhaust at the ridge, because airflow through the attic is limited by whichever side is smaller.

These are the same figures used in our attic ventilation science article, which covers why the physics works this way. This page is the tool: enter your attic floor area and get the number in square inches.

The tool

Calculate your requirement

Nothing is sent anywhere. The math runs in your browser and nothing is stored.

Your ventilation requirement

Enter your attic floor area to see the total net free area required, split into intake and exhaust, plus an estimated vent length.

Common calculator mistakes

Five ways this number comes out wrong before anyone gets on the roof.

Measuring the total square footage of the house instead of the attic floor area above the conditioned space.
Forgetting to exclude vaulted or cathedral ceiling sections, which are not attic and are ventilated differently.
Rounding the requirement down instead of up. Ventilation area is a minimum, so round up to the next whole vent.
Assuming every vent product publishes the same NFA per linear foot. They do not — check the spec sheet for the exact product.
Skipping the local-code check before ordering material, when the jurisdiction may enforce a stricter ratio than 1:300.

Ventilation calculator FAQs

Net free area is the actual open area of a vent that air can pass through, after the louvers, baffles, and insect screen are subtracted. A ridge vent that measures 12 inches wide does not give you 12 inches worth of opening — most of that width is material. That is why code is written in net free area and why two products of the same physical size can have very different NFA per foot. Always use the published NFA figure, not the outside dimensions.

Only if it shares the same attic space as the house. An attached garage with its own ceiling plane and its own separated attic is a separate ventilation zone and should be calculated on its own. If the framing is open between the two, the whole connected volume is one attic and the floor area is added together. When in doubt, look for a firewall or a full partition at the shared wall running to the roof deck.

That is the normal finding, and intake is almost always the side that comes up short. The correction is usually to add soffit intake or clear what is blocked, rather than to cut more exhaust. Adding exhaust to an attic that is already short on intake makes the imbalance worse, because the exhaust then pulls its makeup air through ceiling penetrations instead of the soffits.

Use the conditioned footprint of the floor directly below the attic, taken from the outside dimensions of the house or from your plans, then subtract any part of that footprint that is under a vaulted or cathedral ceiling. That gets you within a few percent without crawling the entire space. If the roof has multiple separate attic sections, measure and calculate each one on its own.

Sometimes. Texas jurisdictions adopt the IRC on their own schedule and can amend it, so the ratio, the vapor retarder condition, and the placement rules for upper and lower vents are all things a city can tighten. Call the building department in the jurisdiction the property sits in and ask which code year they enforce and whether there are local ventilation amendments before you buy material.

Because the airflow through the attic is limited by whichever opening is smaller. Extra exhaust above what the intake can feed does not move more air; it just increases the suction on every other opening in the ceiling. A balanced split gets the most flow from the same total net free area, which is why the balance rule appears in both the code language and most ridge vent warranties.

The intake number still applies, and it matters more, not less. A powered fan moves air whether the intake exists or not, so an undersized soffit area means the fan pulls conditioned air out of the house through can lights and the attic hatch. If you use a powered exhaust, size the intake to the fan's rated airflow per the manufacturer, and do not run it alongside a ridge vent — the two short-circuit each other.

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How much attic ventilation does your roof actually need?

Net free area is the number that matters

Calculate your requirement

Common calculator mistakes

Ventilation calculator FAQs

What is net free area (NFA) and why isn't it the same as vent size?

Does my garage attic count toward the square footage?

What if my calculated intake and exhaust don't match my current vents?

How do I measure attic floor area if I can't access the whole attic?

Is my local code stricter than the IRC 1:300 default?

Why split ventilation 50/50 between intake and exhaust?

Can I mix this calculator's numbers with a mechanical (powered) exhaust vent instead of ridge vent?

Related reading

Let's put your calculated ventilation plan on paper.