Calculate length of ductwork needed. This can be done on a computerized model or by hand utilizing a tape measure and a calculator.
Calculate heat or cooling load. Depending on whether the duct is distributing hot or cool air, or simply ventilating the air inside the room, the gauge, or thickness of the duct, will need to be sized appropriately. What also needs to be figured into this are the number of occupants and their activity within the room. This may require insulated ducts which add to the thickness of each duct, as well as its diameter.
Calculate circulated mass of air needed to ventilate a room. Determine the size of the room. The larger the room, the larger the diameter duct. The volume of air able to be moved through the spiral duct system needs to meet or exceed the volume of air held by the room. It is important to check your local municipal building code to establish a minimum rate at which a room must be ventilated. Also distributors of spiral ductwork will have a chart listing the volume of air each gauge will be able to distribute.
Calculate HVAC mechanical size. Properly sizing the heating, ventilation, and air conditioning unit of the building is important when creating an efficient spiral duct system. Because these calculations factor into many of the calculations concerning air flow, heating and cooling, these calculations must be accurate.