What is the attic ventilation calculation formula?
Short answer. The standard formula is attic floor area in square feet divided by 150, which gives the required net free vent area in square feet; multiply by 144 to convert to square inches. If 40 to 50 percent of that area is exhaust placed high in the attic, the code allows dividing by 300 instead.
The formula step by step
- Attic area: length times width of the attic floor, in square feet. Measure the ceiling area below the attic, not the roof surface.
- Total net free area (NFA): attic area divided by 150, or by 300 if the balanced, high exhaust conditions are met.
- Convert: NFA in square feet times 144 equals square inches, because vents are rated in square inches.
- Split: divide between intake (low) and exhaust (high). The 1/300 path requires 40 to 50 percent exhaust; many manufacturers recommend at least as much intake as exhaust.
- Count vents: divide each half by the NFA rating printed on the product.
Worked example
A 1,200 square foot attic:
- At 1/150: 1,200 divided by 150 is 8 square feet, or 1,152 square inches in total.
- At 1/300: 1,200 divided by 300 is 4 square feet, or 576 square inches, about 288 intake and 288 exhaust.
- With a ridge vent rated 18 square inches per foot, 288 divided by 18 is 16 feet of open ridge.
- With soffit vent rated 9 square inches per foot, 288 divided by 9 is 32 feet of vented soffit.
What the formula does not tell you
NFA means the open area after screens and louvers, not the outside size of the vent, so always use the printed rating. The formula assumes a single connected attic; separate attic spaces over a garage or addition need their own calculation. In Georgia's climate zones the vapor retarder condition in the code does not apply, so the high exhaust placement is the condition that matters. For why all this matters, see roof ventilation basics.