How to Calculate Attic Vent Area Needed When Adding Soffit Vents

By Ben Erickson from todayshomeowner.com  Keeping your attic cooler in the summer can increase the life of the roof as well as saving money on your air conditioning bill. To effectively cool the attic, outside air needs to circulate through it. One solution is to use the natural circulation caused by hot air rising to bring fresh air into the attic through soffit vents under the eaves, then expel the hot air through ridge or gable vents near the peak of the roof.
The general rule of thumb on the amount of total attic vent space needed is to have at least one square foot of vent space for every 150 square feet of attic area. Ideally, half the vents should be located in the soffit at the bottom of the roof and half in gable or ridge vents near the top to allow for natural circulation of air through the attic.
To find out how many soffit vents you need to install:
 Calculate the total vent area needed: Multiply the length of the attic times the width in feet to find the attic area, then divide by 150 to find the total square feet of vent space needed. [(length x width of attic in feet) ÷ 150 = total sq. ft. vent area]
Example: a 50′ x 30′ attic would have a total area of 1,500 sq. ft., divided by 150 equals 10 sq. ft. of total vent space needed.  Calculate the soffit vent area needed: Divide the total vent area by two to determine the soffit vent area. [Vent space ÷ 2 = sq. ft. soffit vent area]
Example: 10 sq. ft. vent area ÷ 2 = 5 sq. ft. soffit vent area.  Calculate the area of each vent: When known, use the “net free area” supplied by the manufacturer of the venting you will be using, which takes into account the actual open area of the vent rather than the total vent size. For square or rectangular vents, muliply the length times width of the vent space in inches, then divide by 144 to convert into sq. ft. [(l x w in inches) ÷ 144 = area of vent in sq. ft.]
Example: a 6″ x 12″ vent would equal 72 sq. in., divided by 144 equals an area of 0.5 sq. ft. per vent.
For round vents, multiply the radius of the vent (half the diameter) times itself (squared), then multiply the total by 3.14 (pi) and divide by 144 to find the number of square feet [(r² in inches x 3.14) ÷ 144 = sq. ft. area per vent].
Example: a 6″ diameter vent would have a radius of 3″, multiplied times itself would equal 9″, times pi (3.14) would give 28.26 sq. in., divided by 144 equals an area of 0.196 sq. ft. per vent.  Determine the number of soffit vents needed: Divide the total soffit vent area by the area of each vent. [soffit vent area in sq. ft. ÷ individual vent area in sq. ft. = number of vents needed].
Example: 5 sq. ft. soffit vent area divided by 0.5 sq. ft. vent area equals 10 soffit vents needed.
Space the soffit vents evenly around the bottom of the low sides of your roof.
There are several different types of soffit vents available including continuous, circular, and perforated vents made for vinyl siding. We’ll discuss the easiest to install, standard 8″ x 16″ vents.
 First mark off the space where you want the soffit vents to go, spacing them so they fit between joists or rafters.
 Cut the hole slightly smaller than the vent itself with a circular saw or a saber saw. Don’t forget to wear eye protection.
 Check to be sure that the hole vents into the attic and isn’t blocked by insulation or other obstructions.
 Screw or nail the vent into place.
The fresh air the soffit vents draw in should be expelled near the peak of the attic through vents in the gables, ridge vents in the roof, wind turbines, or power vent fans.
Originally published at: Today's Homeowner