How much attic ventilation do I need, and how does an attic ventilation calculator work?
Short answer. To calculate attic ventilation, divide the attic floor area by 300, multiply by 144 to get square inches of net free area, and split the result evenly between intake and exhaust; use 150 instead of 300 if your exhaust is not high on the roof. The quick table below gives the answers for common attic sizes.
The two minute calculator
- Attic area: multiply the length and width of the attic floor. For an L shaped house, split it into rectangles and add them.
- Total vent area: attic area divided by 300 (or 150), times 144, equals square inches.
- Intake: half the total, low in the soffits.
- Exhaust: half the total, high on the roof.
- Products: divide each half by the net free area printed on the vent you plan to use.
Ready answers at 1/300, balanced
- 1,200 square feet: 576 square inches total, 288 intake and 288 exhaust, or about 16 feet of ridge vent rated 18 square inches per foot.
- 1,600 square feet: 768 total, 384 each, about 22 feet of that ridge vent.
- 2,000 square feet: 960 total, 480 each, about 27 feet.
- 2,400 square feet: 1,152 total, 576 each, 32 feet.
- 3,000 square feet: 1,440 total, 720 each, 40 feet.
Using the 1/150 ratio, double every number above.
Common input mistakes
- Using the roof surface area instead of the attic floor area. Roof area is larger and overstates the need.
- Using a vent's outside dimensions instead of its net free area rating.
- Counting gable vents and a ridge vent together as if they cooperate.
- Forgetting separate attic spaces, such as over a garage, that do not connect to the main attic.
For the reasoning behind the numbers, see roof ventilation basics and how roofers measure a roof.