Converting an area to square feet would seem simple, unless the area is spread over a form with height, width and depth. Then, you have to convert the outside of that form, its perimeter and length into a workable flat surface and convert the area of that flat surface to square feet. It's a simple enough process as long as you remember the formula.
Measure one side of the duct from top to bottom, then measure the bottom of the duct from left to right, if the duct is square or rectangular. Use a measuring tape that measures in inches. Use the calculator to add these two measurements together and multiply by two. This tells you the length of the perimeter of the duct.
Measure the length of the duct--in inches--and multiply that measurement by the length of the perimeter of the duct. This will tell you how many square inches are in the duct area. Divide this result by 144, the number of square inches in a square foot. This will tell you how many square feet are in the duct area.
Measure the distance across the duct, in inches, then multiply by 3.14159 if the duct is circular to find its circumference, in inches. Then, measure the length of the duct--in inches--and multiply by the circumference. This is the number of square inches in the duct area. Divide by 144 to determine how many square feet are in the duct area.