Attic vent area = attic floor area ÷ 150, or ÷ 300 where the code allows it. A 1,500 sq ft attic needs 10 sq ft, or 1,440 sq in, of net free vent area under the 1/150 rule: half (720 sq in) at the soffits and half at the ridge, which is 40 ft of an 18 sq in per ft ridge vent and 80 ft of a 9 sq in per ft continuous soffit vent.
How attic ventilation is calculated
The 2021 International Residential Code, R806.2, sets the minimum net free ventilating area at 1/150 of the area of the vented space. Net free area is the open area a vent leaves after its screen and louvers. Divide the attic floor area in square feet by 150, then multiply by 144 to get square inches, because vents are rated in square inches. Then split it between low vents (intake) and high vents (exhaust).
Net free area (sq in) = attic floor area (sq ft) ÷ 150 × 144 · Intake = Exhaust = half, when balanced · Ridge vent length = exhaust ÷ NFA per foot
Worked example: 1,500 ÷ 150 = 10 sq ft, and 10 × 144 = 1,440 sq in. Half is 720 sq in of exhaust: a shingle-over ridge vent rated 18 sq in per ft needs 720 ÷ 18 = 40 ft. The other 720 sq in is intake: 720 ÷ 9 = 80 ft of continuous soffit vent, or 720 ÷ 56 = 12.9, rounded up to 13, undereave vents of 16 × 8 in.
1/150 or 1/300
The 1/300 rule halves the requirement: the same attic needs 720 sq in and 20 ft of ridge vent. The 2021 code allows it under conditions: a Class I or II vapor retarder on the warm side of the ceiling in Climate Zones 6, 7 and 8, or 40 to 50 percent of the area high in the attic (within 3 ft below the ridge) with the rest low. If neither applies, use 1/150, and use it anyway when the attic is complicated or the house is airtight, because more vent area does no harm. Air Vent's own guide recommends 1/150 for today's tighter homes. Ask the building department which edition your area has adopted.
| Attic floor area | 1/150 total | 1/150 ridge vent at 18 sq in per ft | 1/300 total | 1/300 ridge vent |
|---|---|---|---|---|
| 1,000 sq ft | 960 sq in | 26.7 ft | 480 sq in | 13.3 ft |
| 1,500 sq ft | 1,440 sq in | 40.0 ft | 720 sq in | 20.0 ft |
| 2,000 sq ft | 1,920 sq in | 53.3 ft | 960 sq in | 26.7 ft |
| 2,500 sq ft | 2,400 sq in | 66.7 ft | 1,200 sq in | 33.3 ft |
Ridge vent, box vents and soffit vents
The net free area of a vent is a product number, so the calculator carries the common ones from the makers' published tables and lets you switch. Air Vent lists its shingle-over ridge vent at 18 sq in per ft, a roof louver (box vent) at 50 sq in, and a 12 in wind turbine at 112 sq in. Cor-A-Vent's V-600 series is 20 sq in per ft and its V-300 series 13.5. On the low side Air Vent's table gives a continuous soffit vent at 9 sq in per ft, undereave vents at 56, 42 and 28 sq in for 16 × 8, 16 × 6 and 16 × 4 in, and perforated aluminum soffit at 14 sq in per sq ft.
Choose Box vents to count roof louvers for the whole exhaust: 720 ÷ 50 = 14.4, so 15. Choose Both when the ridge is short: enter the ridge length you have and box vents cover the rest. With 24 ft of ridge vent that is 432 sq in, and 288 sq in remain, so 6 box vents.
Ridge vent and roof ventilation questions
How much ridge vent do I need? Exhaust area ÷ the vent's NFA per foot, as above: 40 ft for a 1,500 sq ft attic at 1/150. It cannot exceed your ridge, which is why short ridges and hip roofs (where the ridge is shorter than the house) need box vents to make up the difference.
Do I need intake as well? Yes. Air Vent's guide asks for intake area at least equal to the exhaust area, so keep the low half no smaller than the high half. Size the soffit side with the soffit calculator, which shows what share of your soffit has to be vented.
Does the pitch matter? Not for the vent area: the code rule uses the floor area of the vented space, not the roof pitch.
What to work out next
If the attic is being insulated, ventilate first and then count the bags in the blown-in insulation calculator or the insulation calculator, keeping the vent path clear (R806.3 asks for a gap at the eave). For a powered fan or bath exhaust the CFM calculator sets the airflow. Cutting the ridge for a vent comes with a new roof: see the roofing calculator and every other tool in the roofing and siding calculators.