How to Calculate Roof Ventilation
The Federal Housing Administration requires that for every 300 square feet of attic space you have, you must have at least 1 square foot of ventilation. Each vent actually takes up 2 square feet of the requirement, since the outtake and intake of each vent takes up 1 square foot each. The number of vents determined by this method of calculation is the minimum number of vents allowed.

Calculate the size of your attic in square footage:
Length x width = attic size
30' x 40' = 1200 square feet

Use the 1/300 rule to determine the amount of square inches of ventilation required for proper exhaust. Divide the square footage by 300:
1200 / 300 = 4 square feet
Divide the number of square feet by 2 since each outtake and intake takes up 1 square foot of the requirement. In this example, 4 / 2 = 2 square feet.

Convert the square footage into square inches. Multiply the square footage in step 2 by 144:
2 x 144 = 288 square inches

Divide the number of square inches by the area of the vent size to determine how many vents are needed:
288 / 50 = 5.76 vents
Round up to the nearest whole number. In this example, you need 6 vents for proper ventilation.
Things You Will Need
 Calculator
Tip
 Vents come in different sizes. Make sure to divide the number you receive in step 3 by the size in square inches of the vent.