Roofing & Gutters
Attic Ventilation Calculator
Enter the attic floor area, pick the 1/150 or 1/300 rule, and type the net free area printed on your intake and exhaust vents. You get the total vent area and the number of low and high vents.
Worked example
- Net free area = 1,500 sq ft / 300 = 5.00 sq ft = 720 sq in.
- Split about half low (soffit) and half high (ridge or roof vents): 360 sq in each.
- Divide by the net free area printed on each vent: 8 intake and 8 exhaust vents.
Common sizes at a glance
| Size | Total net free area | Intake (low) | Exhaust (high) |
|---|---|---|---|
| 1,000 sq ft, 1/300 | 480 sq in (3.33 sq ft) | 240 sq in: 5 vents | 240 sq in: 5 vents |
| 1,000 sq ft, 1/150 | 960 sq in (6.67 sq ft) | 480 sq in: 10 vents | 480 sq in: 10 vents |
| 1,500 sq ft, 1/300 | 720 sq in (5.00 sq ft) | 360 sq in: 8 vents | 360 sq in: 8 vents |
| 1,500 sq ft, 1/150 | 1,440 sq in (10.00 sq ft) | 720 sq in: 15 vents | 720 sq in: 15 vents |
| 2,000 sq ft, 1/300 | 960 sq in (6.67 sq ft) | 480 sq in: 10 vents | 480 sq in: 10 vents |
| 2,000 sq ft, 1/150 | 1,920 sq in (13.33 sq ft) | 960 sq in: 20 vents | 960 sq in: 20 vents |
| 2,500 sq ft, 1/300 | 1,200 sq in (8.33 sq ft) | 600 sq in: 12 vents | 600 sq in: 12 vents |
| 2,500 sq ft, 1/150 | 2,400 sq in (16.67 sq ft) | 1,200 sq in: 24 vents | 1,200 sq in: 24 vents |
Which ratio applies
The common U.S. residential rule calls for 1 sq ft of net free vent area for every 150 sq ft of attic floor. The code allows a reduction to 1/300 in some cases, typically when a vapor retarder is installed on the warm side of the ceiling or when enough of the vent area is placed high, near the ridge, with the rest low at the eaves.
Running both rules side by side shows the stakes. A 1,500 sq ft attic needs 720 sq in at 1/300 but 1,440 sq in at 1/150. If you are not sure which your home qualifies for, plan for 1/150 or ask the local building department.
Net free area is on the label
A vent's outside size is not its airflow area. Louvers, screens and baffles block part of the opening, so manufacturers print a net free area, in square inches, on the package. Use that figure, not the size of the hole.
Ridge vent is rated per linear foot instead of per piece. Multiply its rating by the feet of ridge you can vent to get its total.
Balancing low and high vents
Air should enter low at the soffits and leave high at the ridge or roof vents. The calculator splits the area about half and half, which gives 8 intake and 8 exhaust vents of 50 sq in each in the example.
Too much exhaust and too little intake can pull air from the house below. Intake should never be the smaller side.
- Keep insulation from blocking soffit vents; baffles help
- Do not mix a ridge vent with gable or power vents on the same attic without checking the vent maker's guidance
- Bathroom and kitchen fans must vent outdoors, not into the attic
How it ties into the rest of the roof
Vent work usually happens during a reroof, when the deck is open. The shingle calculator covers roofing material, and the blown-in insulation calculator helps if you are topping up the attic floor at the same time. The local building code and a licensed professional have the final word on roof and attic work.
Questions
How many soffit vents do I need for a 2,000 sq ft attic?
At 1/300 with 50 sq in vents, 10 intake and 10 exhaust. At 1/150 that doubles to 20 of each.
Do I measure the attic floor or the roof surface?
The attic floor, which is roughly the ceiling area of the rooms below. The roof surface is larger and would overstate the need.
Does a cathedral ceiling use this rule?
A vented cathedral ceiling needs an open air channel in every rafter bay from eave to ridge, so placement matters as much as the total. Have a local roofing or building professional check the design.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.