Attic Ventilation Calculator
Enter the attic floor size. You'll get the net free vent area the building code calls for, split evenly between intake and exhaust, and how many feet of ridge and soffit vent, or how many individual vents, that takes.
- 2021 IRC R806.2
- Manufacturer vent ratings
- Feet or metric
2 Vents you need
Updates as you type| Attic floor area | 1,200 sq ft |
|---|---|
| Code ratio | 1/150 (2021 IRC R806.2 minimum) |
| Total net free area needed | 1,152 sq in, half exhaust and half intake |
| Exhaust (upper) | 576 sq in ÷ 18 sq in per ft = 32 ft of ridge vent (576 sq in installed) |
| Intake (lower) | at least 576 sq in ÷ 9 sq in per ft = 64 ft of continuous soffit vent (576 sq in installed, never less than the exhaust) |
Net free area differs between products, so check the label on the vents you buy and divide by its number. Use one type of exhaust vent per attic: Air Vent warns that mixing a ridge vent with gable vents, louvers or a fan short-circuits the airflow, and DOE says the same about gable vents.
Shopping list
Quick answer
The 2021 IRC calls for 1 square foot of net free vent area per 150 square feet of attic, or 1 per 300 when 40 to 50% of it is in vents near the ridge with the rest low in the attic and, in climate zones 6 to 8, a vapor retarder is installed. A 40 × 30 ft attic (1,200 sq ft) needs 1,152 sq in at 1/150: half intake and half exhaust, or about 32 ft of ridge vent rated 18 sq in per ft and 64 ft of soffit vent rated 9 sq in per ft.
Attic ventilation for common attic sizes
| Attic floor | Vent area at 1/150 | Ridge + soffit at 1/150 | Vent area at 1/300 |
|---|---|---|---|
| 600 sq ft | 576 sq in | 16 ft + 32 ft | 288 sq in |
| 800 sq ft | 768 sq in | 22 ft + 44 ft | 384 sq in |
| 1,000 sq ft | 960 sq in | 27 ft + 54 ft | 480 sq in |
| 1,200 sq ft | 1,152 sq in | 32 ft + 64 ft | 576 sq in |
| 1,500 sq ft | 1,440 sq in | 40 ft + 80 ft | 720 sq in |
| 1,800 sq ft | 1,728 sq in | 48 ft + 96 ft | 864 sq in |
| 2,000 sq ft | 1,920 sq in | 54 ft + 108 ft | 960 sq in |
| 2,500 sq ft | 2,400 sq in | 67 ft + 134 ft | 1,200 sq in |
Net free area at 1/150 and 1/300 (2021 IRC R806.2), split half intake and half exhaust. Ridge vent at 18 sq in per ft, continuous soffit vent at 9 sq in per ft (Air Vent). Use the calculator above for your exact size.
The building code rule
Section R806.2 of the 2021 International Residential Code sets the minimum net free ventilating area at 1/150 of the area of the vented space. The code allows 1/300 when both of these 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.
- Not less than 40% and not more than 50% of the required vent area is in upper vents no more than 3 ft below the ridge, measured vertically, with the rest in the bottom one-third of the attic.
Outside climate zones 6 to 8, only the second condition applies. Find your zone with the climate zone lookup. Your local building department has the final say, since many areas amend the code.
Worked example
A 40 × 30 ft attic floor is 1,200 sq ft. At 1/150, it needs 1,200 ÷ 150 = 8 sq ft of net free area, which is 8 × 144 = 1,152 sq in. Air Vent and GAF size half for intake and half for exhaust, so each needs 576 sq in:
- Ridge vent rated 18 sq in per foot: 576 ÷ 18 = 32 ft.
- Continuous soffit vent rated 9 sq in per foot: 576 ÷ 9 = 64 ft.
Each is rounded up to whole feet or vents, and the intake is never sized smaller than the exhaust actually installed. At 1/300 the same attic needs half as much: 576 sq in. A ridge vent can only be as long as the ridge, less 6 inches at each end in Air Vent's instructions, so on a short ridge or a hip roof you may need a vent with a higher rating.
Net free area of common vents
"Net free area" is the open area air can actually pass through, after screens and louvers. These ratings are from the Air Vent 2026 ventilation catalog:
| Vent | Net free area |
|---|---|
| Ridge vent, 12 in wide (ShingleVent II) | 18 sq in per ft |
| Ridge vent, 9 in wide (ShingleVent II-9) | 16 sq in per ft |
| Roof louver ("box" vent), RVG 50 | 50 sq in each |
| Continuous soffit vent, aluminum | 9 sq in per ft |
| Continuous soffit vent, plastic | 8.2 sq in per ft |
| Flush-mount soffit vent, 16 × 8 in | 58 sq in each |
| Flush-mount soffit vent, 16 × 6 in | 40 sq in each |
Other brands differ: GAF's catalog rates its Master Flow 16 × 8 in soffit vent at 50 sq in. The number printed on the product you buy is the one to use.
Balance intake and exhaust
- GAF: "The amount of exhaust ventilation should NEVER exceed the amount of intake ventilation."
- Air Vent warns against mixing two types of exhaust vent, such as a ridge vent with a powered fan, roof louvers, gable vents or a turbine, on one attic, because air short-circuits between them instead of flowing up from the soffits. DOE's Building America Solution Center gives the same warning for gable vents used with ridge vents.
- Keep insulation off the soffit vents. IRC R806.3 requires at least 1 inch of space between the insulation and the roof sheathing at the vent; baffles (rafter vents) hold it open.
What ventilation does, and doesn't, fix
DOE's Building America Solution Center says a vented attic keeps the roof deck cold in winter, which helps prevent ice dams, and helps dry out leaks and condensation. University of Minnesota Extension adds that ventilation removes heat buildup in summer. Both put air sealing first: the main defense against attic moisture and ice dams is stopping warm house air from leaking into the attic. See how to prevent ice dams.
Frequently asked questions
How much attic ventilation do I need?
The 2021 IRC requires 1 sq ft of net free vent area for every 150 sq ft of attic, or 1 for every 300 when 40 to 50% of it is in upper vents near the ridge with the rest low in the attic and, in climate zones 6 to 8, a vapor retarder is installed in the ceiling.
How many feet of ridge vent do I need?
Half the required vent area in square inches, divided by the ridge vent's rating. For a 1,200 sq ft attic at 1/150, that is 576 sq in ÷ 18 = 32 ft of a vent rated 18 sq in per foot.
Should intake equal exhaust?
Air Vent and GAF both size a balanced system, half intake and half exhaust, and GAF says exhaust should never exceed intake.
Can I have a ridge vent and gable vents?
Air Vent and DOE advise against it on the same attic: the gable vents short-circuit the airflow, so air enters at the gable and leaves at the ridge instead of drawing in through the soffits.
Last reviewed 2026-11-05. See how we calculate for our sources and assumptions.