How to Determine the Number of Attic Vents

Determine the actual opening of the vent when making the calculations. A 1-foot-by-1-foot vent may include screens, louvers or other frame pieces that reduce the effective open area of the vent. On average, about 70 percent of the vent surface is physically open. Place half the square footage of the vents on the soffits, with the rest placed higher on the roof or on the ridge of the home.

Calculate the square footage of the floor space of the attic. In many homes this is the same as the floor space of the home. Determine the square footage by multiplying the length times the width, both stated in feet.

Divide the square footage of the attic by 150. This is the total square footage of ventilation required for the home. Plan to place half the ventilation area in the soffits of the home and half higher on the roof or on the ridge of the roof.

Divide the size of the vent by the total square footage of vents necessary. This determines the number of vents required for the attic.