The code asks for a fraction of the attic floor, in square inches of net free area, not in the size of the hole in the soffit.
1/150default ratioIRC R806.2 requires net free ventilating area of 1/150 of the vented space, with an exception permitting 1/300 when the upper portion and intake are balanced. Confirm the edition your jurisdiction adopted.
The short answer
A 1,200 sq ft attic at 1/150 needs 1,152 sq in of net free area, split 576 into the upper portion and 576 into eave intake. At 18 sq in net free area per vent that is 32 intake vents and 32 exhaust units. Net free area is the number on the datasheet, never the opening size.
Net free area, then product counts
Attic floor area and the ratio. Datasheet net free area per unit. Nothing is emailed.
—Total net free area
—Upper portion (50%)
—Eave intake (50%)
—Exhaust units
—Intake vents
What this assumed—
Arithmetic on R806.2, not a design. Confirm the adopted code and the datasheet. HyreRoof does not install ventilation.
The rule, and the exception that halves it
The baseline requirement in the model residential code is 1 square foot of net free ventilation area for every 150 square feet of attic floor: the 1:150 rule.
That halves to 1:300 where two conditions are met: a vapour retarder is installed on the warm side of the ceiling in colder climates, or the ventilation is balanced with between 40% and 50% of the total in the upper portion of the attic and the remainder as intake at the eaves.
HyreRoof analysis: almost every quote you receive will assume 1:300, because almost every roof is balanced ridge-and-soffit. That assumption is usually fine and occasionally wrong, and when it is wrong the attic is under-ventilated by half. The calculator above runs both so you can see the gap rather than take the assumption on trust.
Net free area is not the size of the hole
Vent type
Typical net free area
What reduces it
Continuous ridge vent
About 18 sq in per linear foot
External baffle design and the width of the slot cut in the deck
Continuous soffit vent
About 9 sq in per linear foot
Insulation pushed into the eave: the single most common defect
Individual soffit vent
About 5-8 sq in each
Paint. Repeated repainting can close a perforated vent almost entirely
Box or louvre vent
About 50-60 sq in each
Screening and louvre geometry
Gable vent
About 70-110 sq in each
Works against ridge-and-soffit flow rather than with it
Turbine
About 100-150 sq in each
Requires wind; a stalled turbine is a hole with a hat on it
Net free area is the open area after screening, louvres and baffles are subtracted, always smaller than the opening it sits in.
Manufacturer figures for a specific product are the ones to use. These are typical ranges for sizing a first pass, and the calculator above is built on the same basis.
Balance matters more than total
How the two halves of a vented attic should be split. Intake is deliberately the larger share.HyreRoof presentation of standard ventilation practice.
A vented attic works by moving air from low intake to high exhaust. That only happens if both exist in roughly the right proportion.
The convention is to aim for slightly more intake than exhaust, around 55% intake to 45% exhaust. Deliberately intake-biased, because the failure mode of the opposite is worse.
HyreRoof analysis: exhaust without matching intake does not ventilate the
attic. It depressurises it, and the air is drawn from wherever it can be found, most commonly
the conditioned house below, through ceiling penetrations and the attic hatch.
That pulls heated or cooled air out of the living space and, in winter, moisture into the attic
where it condenses on the underside of the deck. Adding more exhaust to a roof with
blocked soffits makes the problem worse, not better, and it is a very common
misdiagnosis.
What goes wrong in real attics
Insulation blocking the soffits
The most common defect by a wide margin, and often created by an insulation upgrade that pushed batts or blown material into the eaves. Baffles at each rafter bay are the fix, and they are cheap.
Painted-over soffit vents
Decades of repainting can close a perforated aluminium vent almost completely. It looks like ventilation and is not.
Mixing ridge vents with gable vents
Gable vents short-circuit ridge-and-soffit flow: the ridge pulls from the nearest opening, which becomes the gable rather than the soffit. The attic floor stops being ventilated at all.
Powered fans on an unbalanced attic
A powered vent with inadequate intake will pull conditioned air out of the house. It is the most energetic version of the depressurisation problem.
Bathroom fans terminating in the attic
Not a ventilation fault so much as a moisture source. They must terminate outside, not into the space you are trying to keep dry.
Assuming a new roof fixed it
A reroof replaces the exhaust. Unless somebody went into the eaves, the intake is exactly as blocked as it was before.
Why this is worth getting right
Ventilation is a warranty condition
Many shingle manufacturers require adequate, balanced ventilation. An under-ventilated attic can void the covering warranty, which makes this a contractual question and not only a performance one.
Heat ages shingles from below
An unventilated attic cooks the deck and the covering above it. Ventilation is among the cheapest ways to protect the roof you just paid for.
Winter moisture is the quieter failure
Warm moist air reaching a cold deck condenses on the sheathing. The result looks like a roof leak and is not one, it is the attic raining on itself.
Ice dams are partly a ventilation problem
Heat escaping into the attic melts snow at the ridge, which refreezes at the cold eave. Ventilation and air sealing address the cause; heat cable addresses the symptom.
Unvented assemblies are a different design
A conditioned or unvented attic with insulation at the roof plane is a legitimate approach and this calculator does not apply to it. It requires deliberate design, not the absence of vents.
The exception, quoted in full, because summaries drop half of it
Nearly every bid assumes the 1/300 ratio. The exception that permits it has four separate requirements, and most descriptions of it mention one.
Intake is the larger half on purpose. The code writes the upper portion as a 40–50 percent band, which means the balance of the required area (50 to 60 percent) is intake, and the code puts it in the bottom third of the space.IRC 2021 R806.2, Minimum vent area, International Code Council. Read at https://up.codes/s/minimum-vent-area on 2026-09-05.
The baseline. IRC 2021 R806.2, Minimum vent area: "The minimum net free ventilating area shall be 1/150 of the area of the vented space."
The exception, in full. "The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met: 1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling. 2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space."
Read the word "both". Read the exception carefully: it says "both of the following conditions". The vapour-retarder condition is written as applying in Climate Zones 6, 7 and 8; the 40–50 percent upper-portion condition is not climate-limited. The three-foot rule and the bottom-third rule are part of the exception too, and both are routinely omitted from summaries of it.
There is a relief valve for framing conflicts. "Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted." That is a permission to install lower where the framing genuinely gets in the way, it is not a general licence to ignore the three-foot rule.
Why any of this matters to you. If the exception does not apply, the requirement is not "a bit more ventilation". It is exactly double. A 1,600 square foot attic needs 768 square inches of net free area at 1/300 and 1,536 at 1/150, and no amount of ridge vent rescues a design that assumed the wrong one.
Model code, locally adopted. Model code. Adoption and amendment are local, and the edition your jurisdiction adopted may differ. Confirm with your building department before relying on any of it.
The counter-intuitive part: adding exhaust to a blocked roof makes it worse
Ventilation is a circuit. Exhaust is the half people can see from the street, so exhaust is the half that gets added, which is exactly the wrong move when the other half is shut. Intake lives at the eave, the same edge the gutter calculator sizes, so it is worth checking both while you are looking at that detail.
Illustrative. The failure people diagnose (not enough exhaust) is the one at the bottom of this ordering.HyreRoof analysis of IRC R806.2 and R806.3. Illustrative ordering; we hold no dataset of attic inspections.
The code provision nobody quotes
IRC 2021 R806.3, Vent and insulation clearance is one sentence long and it is the whole story: "Not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent." And, alongside it: "Blocking, bridging and insulation shall not block the free flow of air."
HyreRoof analysis: this is the most-violated line in the whole ventilation chapter, and it is violated by accident. Blown insulation is installed to depth across the attic floor, the installer runs it out to the eave because that is where the attic floor ends, and the soffit intake disappears under it. Nothing looks wrong from the ground, from the loft hatch, or on the roof. The vents are all still there. They are simply not connected to anything.
Model code, locally adopted. Model code. Adoption and amendment are local, and the edition your jurisdiction adopted may differ. Confirm with your building department before relying on any of it. This section was read at UpCodes on 2026-09-05.
What happens physically when intake is shut
A ridge vent does not push air. It provides a low-pressure path out. Air leaves through it whether or not there is anywhere for replacement air to come from: the attic simply goes to a slightly lower pressure than the space below it until the flow balances.
The space below it is your house. So the replacement air comes through the loft hatch, around recessed light housings, up the plumbing chases, through the gaps around bath fan bodies. In summer that pulls cooled air out of the living space. In winter it pulls warm, humid indoor air up onto a cold deck, where it condenses. The homeowner then finds damp sheathing, calls it a leak, and is sold a roof.
Add a powered fan to that attic and you have made the pressure difference larger and the moisture transport faster. This is the single most common way ventilation money is spent on making a building worse.
Why the fault is so hard to see
Nothing about a blocked intake is visible from outside. The soffit vents are all present and all look open. The ridge vent is new. The only place the fault is visible is from inside the attic, lying at the eave with a torch, looking for daylight at the top of the wall plate, and that is a place most people never go and most reroof crews have no reason to.
It is also why a reroof frequently does not fix a ventilation complaint. A reroof replaces exhaust. The intake is untouched, because the intake is not on the roof.
Symptom, misdiagnosis, and what it usually is
The last column is the point of the table. Most ventilation symptoms present as something else, and the something else is expensive.
What you see
What it gets called
What it usually is
What its absence would tell you
Dark staining or frost on the underside of the sheathing, worst near the ridge, in winter
A roof leak
Indoor moisture condensing on a cold deck. A leak stains around a penetration and follows a path; condensation stains broadly and evenly across bays.
If the deck is clean and dry in February, the attic is either genuinely ventilated or genuinely sealed. Either is fine.
Ice building at the eave and in the gutter every winter
A gutter problem, or a job for heat cable
Heat loss into the attic melting snow higher up, refreezing where the roof overhangs unheated air. Ventilation and air sealing address the cause. Whether your jurisdiction also requires an ice barrier membrane under the covering is a separate question, answered in the ice barrier line.
No ice at the eave in a snow climate is a strong sign the attic is genuinely cold, which is what you want it to be.
Curled, cupped or prematurely brittle shingles on the sunniest slope
A bad batch of shingles
Deck temperature. An unventilated attic keeps the underside of the covering hot for far longer each day than a ventilated one.
Even ageing across all slopes points at age rather than at heat.
A cooling bill that is worse than the neighbours’ in an identical house
An undersized air conditioner
Frequently a depressurised attic pulling conditioned air up through ceiling penetrations, plus a hot attic sitting on the ceiling insulation.
If sealing the hatch and the light housings changes nothing, the attic is probably not the path.
Mould at the top of the wall on the north side of the house
A wall or window failure
Often attic air spilling down into the wall cavity at the top plate, which is the same air-sealing failure that feeds the condensation above.
Clean top plates suggest the ceiling plane is sealed, which is the precondition for ventilation working at all.
A powered attic fan that runs constantly and changes nothing
A fan that is too small
Almost always inadequate intake. The fan is moving house air, not outside air, and a bigger fan moves more house air.
A fan that visibly cools the attic and does not raise the electricity bill has intake behind it.
Ventilation faults are diagnosed by their consequences, and the consequences look like other problems. That is why this table is organised by symptom rather than by cause.
HyreRoof analysis. We hold no dataset of attic inspections and do not inspect roofs; these are the patterns the physics and the code provisions imply, offered so you can ask better questions of someone who is standing in the building.
How to check your own intake without going on the roof
Every one of these is done from inside the house or from the ground. None of them requires you to be on the covering.
1
Count what is there from the ground first
Walk the perimeter and photograph the soffit. Continuous strip vent, individual round or rectangular vents, or nothing at all? Write down the linear feet of soffit that actually carries a vent, not the total length of the eave. Houses with a boxed-in porch, an attached garage or a later extension routinely have long stretches of unvented soffit that nobody has ever noticed.
2
Look at the vents, not past them
Perforated aluminium soffit vents close up with paint. From directly below, a phone photograph at full zoom will show whether the perforations are still holes or have become dimples. This is a five-minute job and it settles a question that otherwise costs a call-out.
3
Go into the attic on a bright day, and turn the torch off
Let your eyes adjust and look toward the eaves. You should see a thin line of daylight at the outer edge of each rafter bay. Bays with no daylight are bays with no intake. Count them. If you cannot safely reach the eave, and in many attics you cannot, because there is no floor out there, do not crawl out; photograph along the bays from the walkway instead.
4
Check whether baffles exist at all
A baffle is a rigid chute stapled between the rafters at the eave that holds insulation back and preserves the one-inch airspace R806.3 requires. In a house that has had blown insulation added without them, they will simply not be there. Their absence is not subtle once you know what you are looking for.
5
Find where the bath and kitchen fans go
Follow every flexible duct you can see. It should leave the building, through a roof jack, a gable louvre or a soffit termination. A duct that ends in the insulation, or that is connected to nothing, is putting a shower’s worth of water vapour into the attic several times a day, and no amount of ventilation is a substitute for terminating it properly.
6
Do not get on the roof to finish the job
HyreRoof position: OSHA regulates employers, not homeowners. But the threshold tells you something a homeowner should act on. The federal government requires a trained worker with equipment and a rescue plan to be protected above six feet. "Get up there and have a look" is advice that asks an untrained person to do, unprotected, what a professional may not do unprotected. Falls from height are the leading cause of death in construction. Inspect from the ground, from a window, from a ladder at the eave, or with a drone or a photograph, not from the roof.
If your attic has no safe access, or the insulation is loose-fill and disturbing it is undesirable, stop and hire an inspection. HyreRoof does not perform inspections and takes no fee from anyone who does.
What a good ventilation line on a quote says
Ventilation is usually one word on a roofing bid. Here is what it should be instead.
It states which ratio it designed to, and why
1/150 or 1/300, named explicitly, with the reason the exception does or does not apply on this house. If the bid does not say, the bidder has assumed 1/300 without checking, which is the assumption that is wrong exactly when it matters.
It quotes net free area, not product length
Forty feet of ridge vent is not a ventilation figure. Forty feet at the datasheet net free area per foot is. Ask for the datasheet number and the arithmetic: the calculator above runs the same sum, so you can check it in a minute.
It includes a line for intake
If the bid adds a ridge vent and says nothing about the soffit, it has priced half a circuit. The correct response is not to reject the bid, it is to ask what they found at the eaves, and if the answer is that they did not look, that is the answer you needed.
It prices baffles per bay where they are missing
Baffles are inexpensive materials and awkward labour. A bid that includes them has been into the attic. A bid that includes them at a suspiciously round count has estimated them, which is fine if it says so.
It does not solve the problem with a powered fan
A powered attic fan on an intake-starved attic is the fault amplified, not corrected. If a fan is proposed, the intake calculation should be in front of it, not behind it.
It does not leave gable vents open alongside a new ridge vent
The ridge will draw from the nearest opening. If that is a gable louvre eight feet away rather than a soffit vent thirty feet away, the attic floor is no longer in the airflow path at all. Either the gables get closed or the ridge vent was the wrong product for this roof.
It does not promise an energy saving figure
Ventilation is a durability and moisture measure first. Anyone quoting you a specific percentage off a cooling bill for a ventilation upgrade is quoting a number they cannot support on your house.
What this calculator cannot do
The ventilation calculator runs one piece of arithmetic honestly. Here is the boundary of what that arithmetic is worth.
What it does do
Converts a vented-space floor area into required net free ventilating area at either code ratio, and splits it into intake and upper-portion halves.
Converts those halves into a product count once you supply the datasheet net free area per unit, which is the step most online ventilation advice skips.
Shows both ratios side by side, so you can see the size of the gap the exception opens rather than accepting a bidder’s assumption about it.
What it does not do
It does not design an unvented assembly. A conditioned attic with insulation at the roof plane is a different building science problem governed by different code sections, and none of the arithmetic here applies to it.
It does not know your attic floor area. The vented space is not the house footprint. Kneewall attics, cathedral sections, tray ceilings and rooms in the roof all change what counts, and getting that wrong changes the answer proportionally.
It does not verify that your intake exists. It sizes what should be there. Whether the soffit is open is a physical question, and the steps above are how you answer it.
It does not know your adopted code. Everything here is model text read at UpCodes. Your jurisdiction may have adopted a different edition or amended the exception, and its answer beats ours.
It is not an air-sealing plan. Ventilation only works above a reasonably airtight ceiling. If the ceiling plane leaks badly, ventilation moves house air, and sealing has to come first.
And it is not the assembly reference. NRCA publishes The NRCA Roofing Manual, the trade’s reference for assembly detailing. It is a purchased document rather than a free publication, so this site does not quote from it and describes only what is publicly and consistently stated about its role. The controlling text is the manual itself, and a competent contractor has a copy.
The vocabulary, once
Net free ventilating area
The open area remaining after screens, louvres and weather baffles are subtracted from an opening. It is a published property of a product, not a measurement of the hole in the building, and it is the only unit the code sizes in.
Vented space
The attic or rafter space the ratio is applied to. Not the house footprint, and not the roof surface. Where a house has several separate attic volumes, each is sized on its own.
Intake
Low ventilation, normally at the soffit or in the bottom third of the space. The code puts the balance of the required area here and requires it to be in the bottom third when the 1/300 exception is used.
Upper portion
The code’s term for the exhaust half, 40 to 50 percent of the required area, within three feet of the ridge unless framing genuinely prevents it.
Baffle
A rigid chute stapled between rafters at the eave that holds insulation back and preserves the one-inch airspace R806.3 requires. Cheap, easily omitted, and the difference between a soffit vent that works and one that is decorative.
Short-circuit
Airflow that leaves through a high opening without having entered at a low one, most often a gable louvre feeding a ridge vent. The attic floor stops being ventilated even though the total open area looks correct.
Depressurisation
The condition of an attic with more exhaust than intake, which draws its replacement air from the conditioned house below through ceiling penetrations. It is the mechanism behind most ventilation upgrades that make a building worse.
Unvented attic assembly
A deliberately sealed roof with insulation at the roof plane and no ventilation openings. A legitimate and well-established approach, governed by its own code provisions, and completely outside the scope of this page.
Vapour retarder
A material limiting vapour diffusion through the ceiling, required on the warm-in-winter side in Climate Zones 6, 7 and 8 as one of the two conditions of the 1/300 exception. It is not the same thing as an air barrier, and the attic needs both for different reasons.
Questions this calculator answers
How much attic ventilation do I need?
IRC R806.2 starts at 1/150 of the attic floor as net free area. A 1,200 sq ft attic is 1,152 sq in. The 1/300 exception only applies when intake and the upper portion are both present and balanced. Confirm your adopted code.
What is net free area?
The actual open area after screens, louvers and weather baffles. A 16-inch soffit vent does not give you 16 inches of ventilation.
Why 50/50?
R806.2 puts the upper portion in a 40–50% band. 50/50 is the least demanding point in that band for intake. If the eaves cannot deliver it, no split the code allows will rescue the design.
Sources and methodology
Figures dated 9 August 2026. Last reviewed .
IRC 2021 R806.3, Vent and insulation clearance (ICC via UpCodes, retrieved 2026-09-05. The one-inch airspace and the requirement that insulation not block the free flow of air. Model code; adoption is local.)
29 CFR 1926.501(b), duty to have fall protection (OSHA, read at the Cornell LII mirror, retrieved 2026-09-05. osha.gov returned HTTP 403 to automated retrieval on 5 September 2026, so the text was read at the Cornell LII mirror and is cited that way.)
IRC 2021 R806.2, Minimum vent area (ICC via UpCodes, retrieved 2026-08-09. Your jurisdiction may have adopted a different edition or amended the exception.)