Attic Ventilation Calculator
Attic ventilation is sized by net free area, the genuinely open area of vents, relative to the attic floor area. The building-code rule gives one square foot of net free area for every 300 square feet of attic when intake and exhaust are balanced, or one per 150 when they are not. This calculator divides your attic floor area by the ratio you choose, then splits the requirement evenly between low intake and high exhaust vents, reporting the result in square feet and square inches.
Attic ventilation formula
Attic floor area = length * width
Total net free area (sq ft) = floor area / ratio denominator
Total net free area (sq in) = net free area sq ft * 144
Intake = exhaust = total net free area sq in / 2
The ratio denominator is 300 for balanced intake and exhaust or 150 otherwise. There are 144 square inches in a square foot. The total net free area is split evenly so half is low intake and half is high exhaust.
Attic ventilation notes
- The 1 to 300 ratio applies when intake and exhaust net free areas are balanced.
- The 1 to 150 ratio (twice the area) applies when balance is not met or code requires it.
- Net free area is the open area after screens, louvers, and frame, less than vent size.
- There are 144 square inches in one square foot; vents are rated in square inches.
- Pair low soffit intake with high ridge exhaust for even airflow up and out.
Attic ventilation: frequently asked questions
How much attic ventilation do I need?
A widely used building-code rule provides at least 1 square foot of net free ventilating area for every 300 square feet of attic floor, when intake and exhaust are balanced. Where that balance is not met, the ratio is 1 to 150. This calculator divides your attic floor area by the ratio denominator you select to give the required net free area.
What is net free area?
Net free area (NFA) is the actual open area of a vent after subtracting the screen, louvers, and frame that block airflow. Manufacturers publish the NFA of each vent in square inches, which is less than the vent's overall size. Match the total vent NFA you install to the required NFA this calculator reports.
Why split intake and exhaust 50/50?
Balanced ventilation pairs low intake vents (such as soffit vents) with high exhaust vents (such as ridge vents) so air flows up and out evenly. Codes that allow the 1 to 300 ratio require roughly half the net free area as intake near the eaves and half as exhaust near the ridge. This calculator splits the requirement evenly.
When should I use the 1 to 150 ratio?
The stricter 1 to 150 ratio (twice as much vent area) applies when the intake and exhaust are not balanced or where local code or climate requires it. If you cannot achieve a balanced split or your jurisdiction specifies it, select 150 as the ratio denominator for the larger required area.
How do I convert the result to vent units?
The calculator gives net free area in both square feet and square inches, since vents are rated in square inches. Divide the required intake square inches by the NFA per intake vent to find the number of intake vents, and do the same for exhaust vents using their rated NFA.
Official sources
- U.S. Department of Energy: Attic Ventilation guidance.
- U.S. National Institute of Standards and Technology: Unit conversion (square foot, square inch).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.