How to Calculate Attic Vents

Attic ventilation is a critical home system. Air circulation under the roof helps balance the temperatures between the attic and outdoors, preventing overheating in all seasons and ridding the attic of moisture that can lead to mold and other problems. Determining the amount of ventilation needed for your attic involves a simple calculation. Once you know how much “net free vent area” is required for your attic space, use this number when shopping for ventilation products and/or services. Net free vent area is the amount of true ventilation area supplied by a vent; generally speaking, it’s the vent opening minus any louvers or other parts of the vent structure.

Instructions

- 1
  Measure the width and length of your roof, from eave to eave (not from eave to peak). Also include the garage if it is attached. Multiply the width and length dimensions to find the square footage of the attic space. As an example, let’s say your roof measures 75 feet by 25 feet. The total square footage is 1,875.
- 2
  Divide the square footage by 300. In our example: 1,875 divided by 300 equals 6.25. This is the recommended area of ventilation for your attic.
- 3
  Convert the ventilation area to square inches (most vent products are rated in square inches) by multiplying by 144. Therefore, 6.25 times 144 equals 900. Your attic requires 900 square inches of net free vent area.
- 4
  Calculate the proportions of intake vents (in the eaves or soffits) and exhaust vents (near the peak of the roof or along the ridge) required. This may be 50 percent intake and 50 percent exhaust (450 square inches for each, in our example) or it may be 60 percent intake (540 square inches) and 40 percent exhaust (360 square inches). Consult your city’s building department for general recommendations on choosing and installing attic vents.

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