What size attic vent do I need?
Short answer. Size attic vents by net free area, not by how big they look: multiply your attic floor area in square feet by 0.48 to get the total square inches needed for a balanced system, then split it about half intake and half exhaust. Divide each half by the rated net free area of the vents you are buying to get the count or length.
Quick reference for a balanced system
- 1,000 sq ft attic: about 480 sq in total, 240 intake and 240 exhaust.
- 1,500 sq ft: about 720 total, 360 each.
- 2,000 sq ft: about 960 total, 480 each.
- 2,500 sq ft: about 1,200 total, 600 each.
- 3,000 sq ft: about 1,440 total, 720 each.
If the vents cannot be balanced between high and low, the code requires double these amounts. Measure the attic floor, not the roof surface, and leave out vaulted rooms or bonus rooms that are not part of the open attic.
Converting to vents
- Ridge vent: often rated somewhere around 12 to 18 square inches per foot. At 18, 480 square inches takes about 27 feet.
- Box vents: often around 50 square inches each, so 480 takes about 10.
- Soffit strip vent: check the per foot rating; lower ratings mean long runs.
- Rectangular soffit vents: use the stamped rating, which is far below the face size.
Replacing a single vent
When replacing one damaged box vent, match or exceed its net free area, keep it at the same height as the others and use the same style so the system stays balanced. See roof ventilation basics and ridge vent vs box vents.