Attic Ventilation Calculator
Enter your attic's floor area and a ventilation ratio to calculate the required net free ventilation area — the combined open area needed across soffit and ridge (or gable) vents.
How the Attic Ventilation formula works
A standard ratio-based guideline:
Required vent area (ft²) = Attic floor area / Ventilation ratio In square inches = Required vent area (ft²) × 144
A 1:300 ratio is a commonly cited general guideline (1 ft² of net free vent area per 300 ft² of attic floor), often split evenly between intake (soffit) and exhaust (ridge or gable) vents.
Step-by-step calculation
- Divide attic floor area by the chosen ventilation ratio (commonly 300, or 150 in certain code situations).
- Multiply by 144 to convert from square feet to square inches, the unit vent products are usually rated in.
- Split the total roughly evenly between intake (soffit) and exhaust (ridge/gable) vents, per common guidance.
Worked example
A 1,500 ft² attic at a 1:300 ratio: Required vent area = 1,500/300 = 5 ft², or 5 × 144 = 720 in² — roughly 360 in² of intake and 360 in² of exhaust.
Frequently asked questions
Why does attic ventilation matter?
Proper attic ventilation helps regulate temperature and moisture, which can reduce ice dam formation in cold climates, ease cooling load in hot climates, and help prevent moisture buildup that can lead to wood rot or mold over time.
When would a 1:150 ratio apply instead of 1:300?
Some building codes require the stricter 1:150 ratio when an attic lacks a vapor barrier or doesn't meet certain other conditions — check local code requirements for your specific situation rather than assuming 1:300 always applies.