Roof Vent Calculator
Calculate how much attic ventilation area you need, split between intake and exhaust vents.
Attic ventilation is sized off the attic's floor area using a simple ratio: 1 square foot of net free ventilating area (NFVA) for every 150 square feet of attic, or 1:300 if the attic has a proper vapor barrier and the ventilation is split evenly between intake and exhaust. That total NFVA is then divided roughly in half — half at the soffits as intake, half at the ridge or roof as exhaust — because air has to come in low and exit high for the system to actually work, not just add vents wherever there's room.
Frequently Asked Questions
What is the 1:150 vs 1:300 rule for attic ventilation?
It is the ratio of attic floor area to required net free ventilating area. Use 1:150 (1 sq ft of NFVA per 150 sq ft of attic) as a conservative default, or 1:300 if the attic has a vapor barrier and the intake/exhaust is properly balanced.
Should intake and exhaust vents be equal?
Yes, most building codes and manufacturers require a roughly 50/50 split between intake (usually soffit vents) and exhaust (usually ridge or static roof vents) so air flows continuously from low to high points in the attic.