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. Most building codes require roof ventilation.
1. Calculate the size of your attic in square footage:

Length x width = attic size

30' x 40' = 1200 square feet

2. 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.

3. Convert the square footage into square inches. Multiply the square footage in step 2 by 144:

2 x 144 = 288 square inches

4. 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.