For non-powered attic vents (ridge vents, roof louvers, wind turbines, etc.)
Building codes give minimum attic ventilation requirements. Most attics require more. In fact, research suggests you need 1 square foot of ventilation space, or net free area, for every 150 square feet of attic floor space. Net free area is the total unobstructed area through which air can enter or exhaust a non-powered vent.

For new home construction with a vapor retarder, the minimum is 1 square foot of ventilation or net free area for every 300 square feet of attic floor space.

If your vents are split between ridge vents and intake vents, the minimum requirement is also 1 square foot of ventilation or net free area for every 300 square feet of attic floor space.

For power attic vents
Power Attic Ventilators are rated in CFM. This number represents the cubic feet of air moved per minute. The higher the CFM, the more powerful the ventilator. According to the Home Ventilating Institute, to determine the correct CFM rating needed, multiply attic square footage by 0.7. For example: 1500 square footage (attic) x 0.7 = 1050 CFM (look for a power ventilator rated 1050 CFM or higher).

When using power vents, be sure to provide sufficient attic intake ventilation to match the exhaust capacity of the fan. To determine this, divide the CFM by 300 for the number of square feet of intake needed. Because most vents are rated in inches instead of feet, simply multiply the number of square feet by 144 to calculate the net free area of intake needed.

For whole-house fans
Whole-House Fans are also rated in CFM. To determine the right size for your house, calculate the interior square footage of living area (do not include garage, attic or basement) and multiply the total square footage by three. This will give you the CFM number to look for when choosing your whole-house fan.