Find how much attic ventilation you need. Enter the attic floor area and how your vents are placed to get the required net free area under the IRC 1/150 or 1/300 rule, split into intake and exhaust, and the number of ridge, box and soffit vents.
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Attic size
Area of the vented attic space (usually the ceiling area below)
Code conditions (IRC R806.2)
Vent ratings (net free area, check the product label)
sq in/ft
ft
sq in
sq in/ft
sq in
e.g. a 16 × 8 in louvered vent
Enter your values to see the result.
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How to use the Attic Ventilation Calculator
Enter the attic floor area, or its length and width, in feet or meters.
Choose the vent placement: a high/low split lets you use the 1/300 rule.
Tick the climate zone box if you live in zones 6–8, and say whether there is a vapor retarder.
Check the vent ratings against the NFA printed on your vent products.
Press Calculate to get the vent area and the number of each vent type.
Attic ventilation formulas (IRC R806.2)
Net free area (sq ft) = attic floor area ÷ 150, or ÷ 300 when the exception applies
Net free area (sq in) = sq ft × 144
Exhaust area = NFA × 40–50%; intake area = the rest
Ridge vent (ft) = exhaust sq in ÷ ridge vent NFA per foot
Number of vents = ⌈area ÷ NFA of one vent⌉
The 1/300 exception (2018 and 2021 IRC) applies when both conditions are met: 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, and 40–50% of the required vent area is in the upper part of the attic, no more than 3 ft below the ridge, with the balance in the lower third (eaves or soffits). Older code editions used "either" instead of "both", so check which edition your area has adopted.
Worked example: a 1,500 sq ft attic with balanced ridge and soffit vents outside zones 6–8. 1,500 ÷ 300 = 5 sq ft × 144 = 720 sq in. Exhaust 360 sq in ÷ 18 = 20 ft of ridge vent. Intake 360 sq in ÷ 9 = 40 ft of continuous soffit vent, or 7 individual 56 sq in soffit vents.
Required vent area by attic size
Attic area
NFA at 1/150
NFA at 1/300
Ridge vent at 1/300 (18 sq in/ft, 50%)
Box vents at 1/300 (50 sq in, 50%)
1,000 sq ft
960 sq in
480 sq in
13.3 ft
5
1,500 sq ft
1,440 sq in
720 sq in
20 ft
8
2,000 sq ft
1,920 sq in
960 sq in
26.7 ft
10
2,500 sq ft
2,400 sq in
1,200 sq in
33.3 ft
12
3,000 sq ft
2,880 sq in
1,440 sq in
40 ft
15
Typical vent ratings
Ridge vent: commonly about 12–18 sq in of NFA per linear foot.
Box / static roof vent: commonly about 35–60 sq in each.
Continuous soffit vent: varies widely, often about 4–10 sq in per foot.
Perforated vinyl or aluminum soffit: rated per square foot of panel; check the label.
Ratings differ between products; always use the net free area printed on the packaging. Make sure insulation baffles keep the soffit vents open to the attic.
Frequently asked questions
How much attic ventilation do I need?
The International Residential Code (R806.2) requires at least 1 sq ft of net free vent area for every 150 sq ft of vented attic. That drops to 1 per 300 sq ft when 40–50% of the vent area is in the upper attic (within 3 ft of the ridge) and the rest is low at the eaves, and in climate zones 6, 7 and 8 a Class I or II vapor retarder is also on the warm side of the ceiling.
How do I calculate net free area for my attic?
Divide the attic floor area by 150 or 300 to get square feet of vent area, then multiply by 144 for square inches. A 1,500 sq ft attic at 1/300 needs 5 sq ft, or 720 sq in, of net free area: 360 sq in of exhaust and 360 sq in of intake for a balanced system.
What is net free area (NFA)?
Net free area is the actual open area air can pass through once louvers, screens and baffles are taken into account. It is always smaller than the vent's outer size, so use the NFA printed on the vent packaging rather than measuring the vent.
How many box vents do I need?
Divide the exhaust vent area by the NFA of one vent and round up. For 360 sq in of exhaust and box vents rated 50 sq in each, you need 8 vents. Product ratings vary from about 35 to over 60 sq in, so check the label.
Should intake and exhaust be balanced?
Yes. Most roofing and vent manufacturers recommend at least as much intake as exhaust, which is why a 50/50 or 40/60 exhaust/intake split is common. Too little intake lets exhaust vents pull conditioned air from the house or draw rain and snow in through other vents.
Can I mix ridge vents and box vents?
Most manufacturers advise against using two types of exhaust vent on the same roof area, because the vent nearer the ridge can pull air in through the other one instead of from the soffits. Choose one exhaust type per attic space and size it to the exhaust area.
How many feet of ridge vent do I need?
Divide the exhaust area by the ridge vent's NFA per foot. At 18 sq in per foot, 360 sq in of exhaust needs 20 ft of ridge vent. Leave the ridge vent stopping short of the gable ends and hips as the manufacturer directs.
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