What is the roof ventilation calculation formula?
Short answer. The basic formula is: required net free area in square inches equals attic floor area in square feet divided by 300, times 144. Use 150 instead of 300 if the vents are not balanced between high and low, then split the result between intake and exhaust and divide by each vent's rated net free area to get vent counts or linear feet.
The formulas
- Total NFA (sq in) = A / R x 144, where A is attic floor area in square feet and R is 300 for a balanced system or 150 for an unbalanced one.
- Exhaust NFA = Total NFA x 0.4 to 0.5, placed within 3 feet of the ridge vertically to qualify for the 1/300 ratio.
- Intake NFA = Total NFA minus exhaust, and in practice at least equal to exhaust.
- Feet of ridge vent = Exhaust NFA / vent rating per foot.
- Number of box vents = Exhaust NFA / rating per vent, rounded up.
- Feet of continuous soffit vent = Intake NFA / rating per foot.
Shortcut
For a balanced system, A / 300 x 144 simplifies to A x 0.48. A 1,500 square foot attic needs about 720 square inches in total, 360 each way.
Where the numbers come from
The ratios are in Section R806.2 of the International Residential Code, and Georgia's mandatory residential code is the 2024 edition with state amendments. Net free area ratings come from the vent manufacturer and account for screens and louvers, which is why they are always smaller than the visible opening.
Practical adjustments
- Count only attic area that is actually connected; separate attic pockets get their own calculation.
- Do not count painted over or insulation blocked vents.
- Do not add extra exhaust to make up for weak intake. Extra exhaust without intake pulls air from the house.
Background in roof ventilation basics.