How much attic ventilation do I need?

The IRC base rule is 1 sq ft of net free area for every 150 sq ft of attic floor, split between intake low and exhaust high. For the 1,200 sq ft example that is 8 sq ft, or 1,152 sq in, about 576 sq in at the soffits and 576 sq in at the ridge. Where the 1/300 exception is met, it is half: 576 sq in.

When can I use 1/300 instead of 1/150?

IRC R806.2 allows 1/300 as an exception. It involves a Class I or II vapor retarder on the warm-in-winter side of the ceiling in climate zones 6, 7 and 8, and having 40% to 50% of the required area high in the attic (no more than 3 ft below the ridge) with the remainder at the eaves. Secondary quotes of the 2018 and 2021 text say both conditions must be met, while an older ICC excerpt says one or more, and some jurisdictions amend it. The calculator cannot check your conditions, so use 1/150 unless your building department confirms 1/300.

What does net free area mean?

Net free area (NFA, also net free ventilating area) is the open area air can actually pass through a vent, after the screen, louvers and baffles take their share. A vent that is 16 × 8 in has a hole bigger than its 56 sq in rating. Always size with the NFA printed on the product, never the size of the opening.

Why split intake and exhaust, and can I favour the intake?

Air enters low at the soffits and leaves high at the ridge, which drives the flow. The code exception wants 40% to 50% of the area high, so the exhaust is never more than half. Favouring intake, for example 60% low and 40% high, is allowed within that range and helps keep the attic from pulling conditioned air through the ceiling. The 40/60 example here is 692 sq in of intake and 461 sq in of exhaust.

How many feet of ridge vent do I need?

Divide the exhaust NFA by the vent's NFA per foot. The example is 576 sq in ÷ 18 sq in per ft = 32 ft, and a 40 ft ridge has room for it. Ridge vent can't run past the ridge, and it should stop short of the ends, so short ridges may need box vents too.

How many box vents or soffit vents do I need?

Divide each side by the vent rating and round up: 576 sq in of exhaust ÷ 50 sq in is 12 box vents, and 576 sq in of intake ÷ 56 sq in is 11 16 × 8 in undereave vents. Box vents and a ridge vent are alternatives, so don't count both.

Do I need to count the gable vents, or a power fan?

This calculator counts ridge, box and soffit vents, the usual pairing. Gable vents work differently and mixing them with a ridge vent can short-circuit the airflow, so many manufacturers advise against it. Check the code and the vent maker before relying on a power fan to replace net free area.

Does the attic floor area include the whole house footprint?

Use the floor area of the attic space that is vented. A separate unvented section, such as a conditioned or sealed area, can be entered as a section to leave out. Cathedral ceilings, rafter bays and unvented roof assemblies follow different rules, so check R806 for those.

Attic Ventilation Calculator: How Much Net Free Area Do I Need?

A 40 × 30 ft attic floor (1,200 sq ft) at the 1/150 rule needs 1,152 sq in (8 sq ft) of net free area: 1,200 sq ft ÷ 150 = 8 sq ft. Split 50/50, that is 576 sq in of exhaust at the ridge, which is 32 ft of 18 sq in per ft ridge vent or 12 box vents of 50 sq in, and 576 sq in of intake at the soffits, which is 11 undereave vents of 56 sq in or 64 ft of continuous soffit vent at 9 sq in per ft. Where the code exception for 1/300 is met, the same attic needs 576 sq in.

Figures based on the sources listed belowUpdated 2026-10-08How we calculate

Attic floor length

Attic floor width

Ventilation ratioIRC R806.2 sets 1 sq ft of net free area per 150 sq ft of attic floor. It allows 1/300 only where the exception is met: a Class I or II vapor retarder on the warm side of the ceiling in climate zones 6 to 8, and 40 to 50% of the area high (within 3 ft of the ridge), with the rest at the eaves. The wording differs by edition, so check the one your area adopted. When in doubt use 1/150.

Intake / exhaust splitThe code exception wants 40 to 50% of the area high, so exhaust never goes above half. Favouring intake keeps the attic from pulling conditioned air through the ceiling. These are the only splits offered.

Ridge vent NFA (sq in per ft)

The rating printed on the product: GAF Cobra SnowCountry is 18 sq in per linear ft, and some other ridge vents are lower. Use your product’s number.

Box vent (roof louver) NFA (sq in each)

Lomanco 750 static roof louvers are rated 50 sq in each; other sizes and makers differ.

Soffit (undereave) vent NFA (sq in each)

Air Vent’s 16 x 8 in aluminum undereave vent is rated 56 sq in; its 16 x 4 in size is 28. Use the rating of the vent you buy.

Continuous soffit vent NFA (sq in per ft)

Air Vent’s continuous aluminum soffit vent (SV201) is rated 9 sq in per linear ft. Perforated vinyl soffit panels vary a lot: check the package.

1,152 sq in (8 sq ft) of net free area

Total net free area: 1,152 sq in (8 sq ft)
Intake (soffit), low: 576 sq in (4 sq ft)
Exhaust (ridge or box), high: 576 sq in (4 sq ft)
Ridge vent for the exhaust: 32 ft, at 18 sq in per ft
Or box vents for the exhaust: 12 vents, at 50 sq in each
Soffit (undereave) vents for the intake: 11 vents, at 56 sq in each
Or continuous soffit vent for the intake: 64 ft, at 9 sq in per ft

Shopping list

Ridge vent (roll or rigid)
Aluminum soffit vents (16 x 8 in)
Continuous soffit vent strip
Box vent (static roof louver)
Attic baffles (rafter vents)
Roofing sealant
Hole saw set (soffit vent holes)

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Net free area by attic size: 1/150 and 1/300

Balanced 50/50 split, worked by this calculator. Square inches of net free area. Exhaust / intake is each side in sq in; ridge vent is at 18 sq in per ft. 1/300 applies only where the code exception is met.
Attic floor	Total at 1/150 (sq in)	Exhaust / intake at 1/150	Ridge vent at 1/150	Total at 1/300 (sq in)	Exhaust / intake at 1/300	Ridge vent at 1/300
30 × 20 ft (600 sq ft)	576	288 / 288	16 ft	288	144 / 144	8 ft
40 × 25 ft (1,000 sq ft)	960	480 / 480	27 ft	480	240 / 240	14 ft
40 × 30 ft (1,200 sq ft)	1,152	576 / 576	32 ft	576	288 / 288	16 ft
50 × 30 ft (1,500 sq ft)	1,440	720 / 720	40 ft	720	360 / 360	20 ft
50 × 40 ft (2,000 sq ft)	1,920	960 / 960	54 ft	960	480 / 480	27 ft
50 × 50 ft (2,500 sq ft)	2,400	1,200 / 1,200	67 ft	1,200	600 / 600	34 ft

Vent ratings used here

Published manufacturer net free area. Other products differ, so use the rating on your package.
Vent	Net free area used	Source product
Ridge vent	18 sq in per linear ft	GAF Cobra SnowCountry Advanced
Box vent (static roof louver)	50 sq in each	Lomanco 750
Soffit (undereave) vent, 16 × 8 in	56 sq in each	Air Vent aluminum undereave vent
Soffit (undereave) vent, 16 × 4 in	28 sq in each	Air Vent aluminum undereave vent (enter 28 to use)
Continuous soffit vent	9 sq in per linear ft	Air Vent SV201 aluminum

The IRC R806.2 ventilation rule at a glance

A summary, not the code text. The wording differs by edition and local amendment: read the edition your building department enforces.
Case	Required net free area	Condition
Base rule	1/150 of the vented attic floor area	Applies unless the exception below is met
Exception	1/300 of the vented attic floor area	A Class I or II vapor retarder on the warm-in-winter side of the ceiling (in climate zones 6, 7 and 8), and 40% to 50% of the area from vents high in the attic, within 3 ft of the ridge, with the rest at the eaves or cornice. Editions differ on whether one or both conditions must be met

Methodology & assumptions

Total net free area (NFA) = attic floor area ÷ ratio, converted to square inches (× 144). The ratio is 150 for the base rule in IRC R806.2, or 300 where the exception for 1/300 is met. The area is the floor of the vented attic (length × width), less any section you enter as unvented. Exhaust (high) NFA is the share you pick of the total, 50%, 45% or 40%; intake (low) NFA is the rest. Those shares keep the exhaust at or below half, the 40 to 50% range the exception sets for the upper vents. Each vent count divides that side's NFA by the NFA rating of the vent and rounds up, and ridge or continuous soffit length is the NFA divided by the NFA per foot, rounded up to the next foot. The default NFA ratings are published manufacturer figures: 18 sq in per ft for a GAF Cobra SnowCountry ridge vent, 50 sq in for a Lomanco 750 box vent, 56 sq in for an Air Vent 16 × 8 in undereave vent and 9 sq in per ft for an Air Vent SV201 continuous soffit vent. Other products differ, so enter the rating on your package; NFA is the open area after the screen and louvers, not the size of the hole. The ridge vent and box vent lines are alternatives for the same exhaust, as are the soffit vent and continuous soffit lines for the intake: do not add them together. Vents must also fit the roof: the ridge vent cannot be longer than the ridge, and soffit vents need clear baffles so insulation does not block them. The code exception wording differs by edition and some places amend it, so confirm with your building department. This is a code-minimum sizing tool, not a moisture or energy design. The metric toggle shows lengths in m; vent ratings stay in square inches because that is how they are published.

How to calculate attic ventilation

Measure the attic floor. Multiply the length and width of the vented attic floor, and subtract any unvented section.
Pick the ratio. Use 1 sq ft of net free area per 150 sq ft of floor, or 1/300 only if the code exception is met in your area.
Convert to square inches. Divide the area by the ratio and multiply by 144 to get the total net free area in square inches.
Split intake and exhaust. Put 50% low and 50% high, or favour the intake with 55/45 or 60/40, keeping the high share between 40% and 50%.
Choose vents. Divide each side by the NFA rating of the vent, rounding up: ridge vent in feet, box vents and soffit vents by count.

Worked examples

Project	Result
40 × 30 ft attic floor, 1/150, balanced 50/50	1,152 sq in (8 sq ft) of net free area: 576 sq in high (32 ft of ridge vent or 12 box vents) and 576 sq in low (11 soffit vents)
40 × 30 ft attic floor, 1/150, intake-favoured 60/40	1,152 sq in (8 sq ft) of net free area: 461 sq in high (26 ft of ridge vent) and 692 sq in low (13 soffit vents)
50 × 30 ft attic floor, 1/300 exception met, intake-favoured 55/45	720 sq in (5 sq ft) of net free area: 324 sq in high (18 ft of ridge vent) and 396 sq in low (8 soffit vents)
30 × 20 ft attic floor with a 10 × 10 ft unvented section, 1/150, balanced	480 sq in (3.33 sq ft) of net free area: 240 sq in high (5 box vents) and 240 sq in low (5 soffit vents)

Frequently asked questions

How much attic ventilation do I need?: The IRC base rule is 1 sq ft of net free area for every 150 sq ft of attic floor, split between intake low and exhaust high. For the 1,200 sq ft example that is 8 sq ft, or 1,152 sq in, about 576 sq in at the soffits and 576 sq in at the ridge. Where the 1/300 exception is met, it is half: 576 sq in.
When can I use 1/300 instead of 1/150?: IRC R806.2 allows 1/300 as an exception. It involves a Class I or II vapor retarder on the warm-in-winter side of the ceiling in climate zones 6, 7 and 8, and having 40% to 50% of the required area high in the attic (no more than 3 ft below the ridge) with the remainder at the eaves. Secondary quotes of the 2018 and 2021 text say both conditions must be met, while an older ICC excerpt says one or more, and some jurisdictions amend it. The calculator cannot check your conditions, so use 1/150 unless your building department confirms 1/300.
What does net free area mean?: Net free area (NFA, also net free ventilating area) is the open area air can actually pass through a vent, after the screen, louvers and baffles take their share. A vent that is 16 × 8 in has a hole bigger than its 56 sq in rating. Always size with the NFA printed on the product, never the size of the opening.
Why split intake and exhaust, and can I favour the intake?: Air enters low at the soffits and leaves high at the ridge, which drives the flow. The code exception wants 40% to 50% of the area high, so the exhaust is never more than half. Favouring intake, for example 60% low and 40% high, is allowed within that range and helps keep the attic from pulling conditioned air through the ceiling. The 40/60 example here is 692 sq in of intake and 461 sq in of exhaust.
How many feet of ridge vent do I need?: Divide the exhaust NFA by the vent's NFA per foot. The example is 576 sq in ÷ 18 sq in per ft = 32 ft, and a 40 ft ridge has room for it. Ridge vent can't run past the ridge, and it should stop short of the ends, so short ridges may need box vents too.
How many box vents or soffit vents do I need?: Divide each side by the vent rating and round up: 576 sq in of exhaust ÷ 50 sq in is 12 box vents, and 576 sq in of intake ÷ 56 sq in is 11 16 × 8 in undereave vents. Box vents and a ridge vent are alternatives, so don't count both.
Do I need to count the gable vents, or a power fan?: This calculator counts ridge, box and soffit vents, the usual pairing. Gable vents work differently and mixing them with a ridge vent can short-circuit the airflow, so many manufacturers advise against it. Check the code and the vent maker before relying on a power fan to replace net free area.
Does the attic floor area include the whole house footprint?: Use the floor area of the attic space that is vented. A separate unvented section, such as a conditioned or sealed area, can be entered as a section to leave out. Cathedral ceilings, rafter bays and unvented roof assemblies follow different rules, so check R806 for those.