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.