Is there a soffit vent size chart for how much intake an attic needs?

Intake needed by attic size

Attic floor area is roughly the footprint of the space below, not the roof surface. These figures are net free area of intake, assuming a 50/50 split with exhaust:

• 1,000 sq ft attic: about 240 sq in at 1/300, or 480 sq in at 1/150
• 1,500 sq ft: about 360 sq in, or 720 sq in
• 2,000 sq ft: about 480 sq in, or 960 sq in
• 2,500 sq ft: about 600 sq in, or 1,200 sq in
• 3,000 sq ft: about 720 sq in, or 1,440 sq in

Which column applies

The residential code allows the 1/300 figure when 40 to 50 percent of the vent area is up high, within 3 feet of the ridge, with the rest at the eaves. If the house has no real high exhaust, or the vents are badly unbalanced, use the 1/150 column. Many manufacturers also recommend that intake equal or exceed exhaust, so rounding intake up is the safe direction.

Turning square inches into vents

Divide the intake figure by the net free area printed on the vent you plan to use, per vent for individual louvers or per linear foot for strip vents and perforated soffit. Then compare the result with the eave length you have. If the math calls for more soffit vent than your eaves can hold, intake vents along the bottom edge of the roof can make up the difference. Remember that screens, paint and insulation can push real airflow below the rating.

For the exhaust half of the equation, see ridge vent vs box vents; for the reasons behind the ratios, see roof ventilation basics.