How to Size a Whole House Attic Fan

Measure the width, length and ceiling height of each room in the house, including bathrooms and the kitchen but excluding closets. Also, measure hallways and stairwells. Write down the dimensions of each space.

Calculate the volume (in cubic feet) of each space by multiplying the width, length and height. For example, a 10-foot by 12-foot room with an 8-foot ceiling has a volume of 960 cubic feet (10 x 12 x 8 = 960).

Add up the volume of all spaces to find the total volume of air space in the house. Divide this number by 3 to find the recommended CFM rating for the whole house fan. For example, if the total volume of the house is 12,000 cubic feet, you should look for a fan rated for at least 4,000 CFM (12,000 divided by 3 = 4,000).

Find the required ventilation area (in square feet) for your attic by dividing the fan’s CFM rating by 750. For example, a 4,000-CFM fan requires at least 5.33 square feet of total vent area in the attic (4,000 divided by 750 = 5.33). Note: This number represents the “net free ventilation area,” not the dimensions of the vent units. The net free ventilation area accounts for any louvers, bug screening and other obstacles to true ventilation; look for this rating when purchasing vent products.