A duct--which resembles a long, flexible tube--is used in a heating, ventilation, and air conditioning system to deliver and remove ventilation, supply, return, and exhaust air. Whether you are installing a new air conditioning system or making modifications to an existing one, knowing the total amount of air flowing through the ducts is an important piece of knowledge. Knowing the total required air volume helps you to determine the correct power for your air conditioning equipment.

Find the diameter of the duct you’re using on the outside of the packaging. For example, you might have 6-inch diameter duct. To convert to feet, divide by 12. In this example, 6 inch/12 is .5 feet.

Divide the diameter from Step 1 by 2, then multiply it by itself and pi. In the example, .5 feet/2 is .25 feet, and .25 feet x .25 feet x pi = .196 square feet. This is the area of a slice of the duct.

Find the length of the duct by finding the duct length per package on the outside of the package, then multiplying by the number of packages you are using. You might be using five packages with 30 feet each, for a total of 150 feet of duct.

Multiply the area of a slice from Step 2, by the length of the duct from Step 4. In the example, .196 square feet x 150 feet = 29.45 cubic feet. This is the volume of your ducts.