Calculate Required Soffit Intake and Attic Ventilation
Size net free ventilation area from attic floor space using the U.S. 1/150 baseline or a qualified 1/300 option, then divide intake and exhaust.
NFA
Attic and vent inputs
Soffit ventilation requirements
How to calculate soffit vent net free area
Net free ventilation area is the actual open airflow area after screens, louvers and baffles are considered. The U.S. baseline calculation divides the ventilated attic floor area by 150. A 1/300 ratio is available only when the conditions required by the applicable code are met; selecting it in this calculator does not prove those conditions.
The total square feet of ventilation is converted to square inches and split between lower intake and upper exhaust using the selected intake share. Balanced systems normally aim near one-half intake and one-half exhaust. The soffit-vent count is the intake requirement divided by the manufacturer’s NFA per vent and rounded upward.
Continuous soffit vents
For a continuous product, divide required intake square inches by the listed NFA per linear foot. The capacity check compares that required length with available eave length. Do not use the vent’s outside dimensions as NFA; use the tested rating printed by the manufacturer.
Should I automatically use the 1/300 attic ventilation ratio?
No. The U.S. 1/150 ratio is the baseline; 1/300 requires specified code conditions and local approval.