What size are roof vents, and is there a roof vent size chart?
Short answer. Roof vents are compared by net free area, not outside size. A typical box or static vent is rated around 50 square inches, shingle-over ridge vents are commonly rated between about 12 and 18 square inches per linear foot, and turbines are sold by throat diameter, often 12 or 14 inches. The exact rating is printed on the product.
Common vent types and how they are rated
- Box or slant-back static vents: physically about a foot or so across, but rated by open area, commonly around 50 square inches each.
- Ridge vents: rated per linear foot, commonly about 12 to 18 square inches per foot depending on the product. They come in rolls or 4 foot sections.
- Turbines: sold by throat diameter, usually 12 or 14 inches. Airflow rises with wind speed, so use the passive rating for sizing.
- Power fans: rated in CFM of airflow rather than area, and sized to the attic.
- Exhaust caps for fans: sized by the duct they connect to, such as 4, 6 or 8 inches, not by attic area.
A quick chart
Under the balanced 1/300 rule, exhaust is roughly half the total, or about 0.24 square inches per square foot of attic floor:
- 1,500 sq ft attic: about 360 sq in of exhaust, roughly 7 box vents at 50 sq in, or about 20 feet of ridge vent rated 18 sq in per foot.
- 2,000 sq ft: about 480 sq in, roughly 10 box vents, or about 27 feet of ridge vent.
- 2,500 sq ft: about 600 sq in, roughly 12 box vents, or about 34 feet of ridge vent.
Read the label
Ratings vary between brands and models, so plug in the real number from the product you will install, then make sure the intake at the eaves matches. See ridge vent vs box vents and roof ventilation basics.