What size soffit vents do I need?
Short answer. Figure the intake first: about 0.24 square inches of net free area per square foot of attic floor under the balanced 1/300 rule, or roughly 480 square inches for a 2,000 square foot attic. Then choose a vent style that fits your soffit and divide by its rating; continuous strip vents and perforated panels spread intake best, while individual louvers suit narrow wood soffits.
Match the style to your soffit
- Vinyl or aluminum soffit: use fully vented panels along the eaves. Multiply the panel's rated net free area per linear foot by your total eave length.
- Plywood soffit, 12 inches or deeper: a continuous strip vent, 2 or 3 inches wide, cut down the middle gives even airflow to every bay.
- Wood soffit with individual vents: rectangular louvers, commonly 16 inches long and 4 to 8 inches wide, placed between rafters. Use enough of them to reach the total.
- Narrow soffits or frieze boards: small round plug vents, 2 to 4 inches across, but you need many because each moves little air.
- No soffit at all: an intake vent along the roof's lower edge.
A worked example
A 2,000 square foot attic needs about 480 square inches of intake. If the house has 140 feet of eave and you choose a strip vent rated, for example, at 8 square inches per foot, you need about 60 feet of strip spread around all sides. With louvers rated at 20 square inches each, you would need about 24. Plug in the real ratings from the products you buy, since they vary.
Before you cut
Check from the attic that there is a clear path above each new vent. If insulation is packed against the deck, add baffles first or the new vents will do nothing. Keep intake vents away from bath fan or dryer outlets, and set them a little in from the fascia so gutter overflow does not run straight into them. See roof ventilation basics and roof anatomy explained.