How many roof vents do you need per square foot of attic?
Short answer. On a balanced system using the 1/300 ratio, you need about 0.24 square inches of exhaust per square foot of attic floor, which works out to roughly one 50 square inch box vent for every 200 square feet of attic, plus the same amount of soffit intake. Without balanced high and low vents, the 1/150 ratio applies and those numbers double.
The per square foot math
The 1/300 ratio gives 144 divided by 300, or 0.48 square inches of total net free area per square foot of attic floor. Split evenly, that is 0.24 exhaust and 0.24 intake per square foot.
- Box vents rated 50 square inches: one per about 208 square feet of attic. A 1,600 square foot attic needs about 8.
- Ridge vent rated 18 square inches per foot: one foot per about 75 square feet. The same attic needs about 21 feet of open slot.
- Intake: about 384 square inches of soffit vent area for that attic.
When the numbers double
If the vents are not split between high and low, for example gable vents only, the code's 1/150 ratio applies: 0.96 square inches total per square foot. That is why gable only attics so often fall short.
Read the label, not the size
Two box vents that look identical can have different net free area, depending on louvers and screens. The rating is printed on the vent or its box. Use it rather than a rule of thumb whenever you can.
Caveats
- Use attic floor area, not roof area, which is larger on steep roofs.
- Do not add exhaust beyond what the intake can feed.
- Count each separate attic space on its own.
- Keep one exhaust type per attic.