How do I determine how much Net Free Area (NFA) is required in my attic?

Most codes require 1 square foot of ventilation per 150 square feet of attic space.

First, determine the number of square feet in your attic. If your house is 20 feet by 30 feet you'll probably have 600 square feet of attic space. Most codes are now requiring 1 square foot of ventilation per 150 square feet of attic space. So, take 600 and divide it by 150 to get 4 square feet of venting area. To convert from square feet to square inches, multiply by 144 to get 576 square inches of venting area. The 576 square inches of venting area should be divided equally between the ridge and the eave. The ridge therefore needs 288 square inches of venting area. Assuming a gabled roof, each eave gets the remaining 288 square inches divided equally, or, 144 square inches along each eave.