How much Attic Ventilation is Needed

Let us start with the simplest answer, It is an attic for which the total of inlet and exhaust Net Free Area (NFA) in square feet is equal to 1/300 of the open attic insulation floor area in square feet.  Unless you’re building codes call for the newer ratio of 1:150 which results in double the amount of NFA.  Let us say your attic floor area is 30 ft. by 50 ft.  That equals 1,500 sq. ft.   Then, 1500/300 yields 5 square feet of total NFA or 2.5 SF each of inlet and exhaust.  These values are minimums and may be exceeded by any amount.  However, if the inlet and exhaust areas are not equal, then the inlet area must be the larger of the two.