The AirHawk B-144 Metal Dome is a 25 inch round roof vent that provides 144 square inches of net free area per piece. Several of these vents are usually required to properly ventilate the attic. They should be placed evenly across the roof.
A guideline to keep in mind with the Air Vent Air Hawk is that homes require 1 square foot of net free area (the total unobstructed area through which air can enter or exhaust a non-powered vent) for every 150 square feet of attic floor space. If balanced between intake and exhaust vents, the requirement is 1 square foot of net free area for every 300 square feet of attic floor space.