How to Measure Gable Vents

Measure the length and width of your attic floor with a tape measure. If you have an odd layout, such as a wall separating the attic, take the width and length measurement for each part of the attic floor.

Multiply the length by the width to determine the square footage of attic space you need to ventilate. For example if you have a 22-foot wide by 30-foot long attic, the total square feet is 660.

Divide your total square footage by 300 to determine the vent area. Typically you want a vent area of one square foot for every 300 square feet of attic space. When dividing, round up to the next whole number. In the example, you want 3 square feet of vent area.

Purchase gable vents that match the net vent area from your calculations. Take the measurements with you when purchasing gable vents. Some vent manufacturers use different factors of square footage to determine the proper size vent. The information is usually on the vent packaging.