Its Probably A Good Idea To First Understand The Type Of Vents Commonly Found On Homes:

Common Types Of Vents Found In The Attic And On The Roof Of Your Home Include: 

• Whirlybird Roof Vents
• Static Vents
• O'Hagen Flat Vents - Commonly Found On Tile Roofed Homes
• Dormer Vents
• Ridge Vents - Run The Entire Legnth Of The Roof At Its Peak
• Gable Side Vents

How To Find Out How Much Net Free Venting Area  You Have In Your Home:

1.   Measure All The Vents You Find In Your Home

If you were to measure (length x width)  of each vent in your homes attic/roof, you would have a ball park idea of how much roof and attic venting your home has.  

2.  Measure In Inches Then Covert To Sq. Ft.

Take The Total Area Of Your Vents In Inches And Convert To Sq. Ft By Dividing By 144

3.  If Your Vents Are Screened - Take Your Total Area And Cut It In Half! 

Since most vents are screened, which can obstruct 1/2 of the air flowing out of the - once you have your gross total area of venting, multiply that number by .5 to get your Net Free Area Of Venting. 

Example: If we take one 12" x 12" gable end vent and multiply the length (12") by the width (12") we would have a total of 144 square inches of venting.  To convert to sq. ft. we would divide by 144 (since there are 144 sq. inches in one square foot).   When we divide 144/144 we see that equals 1 sq. ft.  But since our vent is screened, we would take our 1 sq. ft and cut it in half.  That leave us with .5 sq. ft of net free venting.

Easy Right?