Measure the attic floor and pick your vents. You get the vent area the building code asks for, half of it high and half low, and how many feet of ridge and soffit vent, or how many vents, that is.
Attic ventilation
576 sq in of vents: 16 ft of ridge vent, 32 ft of soffit vent4 sq ft of net free area for 1,200 sq ft of attic (1/300) · half high, half low
Quick answer: the building code (IRC R806.2) asks for 1 sq ft of net free vent area for every 150 sq ft of attic floor, or 1 for every 300 when 40–50% of it is high on the roof and the rest low at the eaves. A 40 × 30 ft attic at 1/300 needs 576 sq in: 16 ft of ridge vent and 32 ft of soffit vent, 16 ft along each eave.
How to use the attic ventilation calculator
- Measure the attic floor: its length and width, the ceiling below it.
- Say where the vents are: high and low (the 1/300 rule), or anything else (1/150).
- Pick the vents: a ridge vent or roof vents up high, a soffit strip or eave vents down low.
- Add the ridge’s length to check it can carry the ridge vent.
The rules behind the numbers
The code: 1/150, or 1/300
IRC R806.2: “The minimum net free ventilating area shall be 1/150 of the area of the vented space.” The exception allows 1/300 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 “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.” The upper vents go “not more than 3 feet (914 mm) below the ridge”; “The balance … shall be located in the bottom one-third of the attic space.”
Net free area
Net free area is the open part of a vent after its louvers and screen, printed on the vent or its sheet. The calculator uses Air Vent’s: its ShingleVent II ridge vent gives “18 square inches per lineal foot” (ICC-ES report ESR-2071), its continuous soffit vents 9 sq in a foot, its 16 × 8″ undereave vents 56 sq in each.
Half high, half low
The calculator splits the area 50/50: inside the code’s 40–50% high, with never less intake at the eaves than exhaust at the ridge.
Vent area by attic size
| Attic floor | 1/300 | Ridge + soffit vent | 1/150 |
|---|---|---|---|
| 800 sq ft | 384 sq in | 10.7 ft + 21.3 ft | 768 sq in |
| 1,000 sq ft | 480 sq in | 13.3 ft + 26.7 ft | 960 sq in |
| 1,200 sq ft | 576 sq in | 16 ft + 32 ft | 1,152 sq in |
| 1,500 sq ft | 720 sq in | 20 ft + 40 ft | 1,440 sq in |
| 2,000 sq ft | 960 sq in | 26.7 ft + 53.3 ft | 1,920 sq in |
Ridge vent at 18 sq in a foot and soffit vent at 9 (Air Vent); the soffit vent’s feet are split between the two eaves.
Net free area of common vents
| Vent (Air Vent) | Net free area |
|---|---|
| ShingleVent II ridge vent | 18 sq in a foot |
| Airhawk 8″ static roof vent | 50 sq in each |
| Continuous soffit vent | 9 sq in a foot |
| 16 × 8″ louvered undereave vent | 56 sq in each |
| 16 × 6″ louvered undereave vent | 42 sq in each |
| 16 × 4″ louvered undereave vent | 28 sq in each |
Four worked examples
A 40 × 30 ft attic, ridge and soffit vents. 576 sq in at 1/300: 16 ft of ridge vent and 32 ft of soffit vent.
The same attic, vents not split high and low. 1/150 doubles it: 1,152 sq in, 32 ft of ridge vent and 64 ft of soffit vent.
Roof vents and 16 × 8″ eave vents. 6 roof vents near the ridge and 6 eave vents, 3 a side.
A 12 ft ridge on a hip roof. It carries only 216 sq in of ridge vent, short of the 288 the top needs: add 2 roof vents within 3 ft of the ridge.
Common attic ventilation mistakes
- Counting a vent’s size, not its net free area. Louvers and screens block a lot of it; read the label.
- All exhaust, no intake. The code puts the balance low, in the bottom third of the attic.
- Taking 1/300 without its conditions. The vents must be split high and low, and zones 6–8 also need the vapor retarder.
- Insulation over the soffit vents. IRC R806.3: insulation “shall not block the free flow of air”, with at least 1″ between it and the roof sheathing at the vent.
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Attic ventilation questions
How much attic ventilation do I need?
1 sq ft of net free area for every 300 sq ft of attic floor when the vents are split high and low (40–50% high), otherwise 1 for every 150 (IRC R806.2). A 1,200 sq ft attic: 576 sq in, or 1,152.
What are the attic ventilation requirements?
IRC R806.2: a net free vent area of 1/150 of the attic floor, or 1/300 when 40–50% of it is within 3 ft of the ridge and the rest in the bottom third, plus a Class I or II vapor retarder in climate zones 6, 7 and 8.
How do I calculate soffit vents?
Take the low half of the vent area and divide by the vent’s net free area: 288 sq in ÷ 9 sq in a foot = 32 ft of continuous soffit vent, 16 ft along each eave, or 288 ÷ 56 = 6 of the 16 × 8″ eave vents.
How do I calculate ridge vent?
The high half of the vent area ÷ the ridge vent’s net free area a foot: 288 sq in ÷ 18 = 16 ft for a 1,200 sq ft attic at 1/300. If the ridge is shorter, add roof vents near the top.
What is net free area?
The open area air can pass through once the louvers and screen are counted, in square inches. The maker prints it: Air Vent’s ShingleVent II ridge vent is 18 sq in a foot.
What is the 1/300 rule?
The code’s exception: the vent area may be 1/300 of the attic floor instead of 1/150 when 40–50% of it is high on the roof and the rest low, and in zones 6–8 the ceiling has a Class I or II vapor retarder.