How to Size a Spiral Duct

Spiral ducts combine light-gauge metal with a spiral lock-seam construction that ensures maximum strength and rigidity.

Size a Spiral DuctSize a Spiral Duct
The spiral duct is a versatile product that comes in a number of thicknesses and shapes to meet all types of projects. Sizing a spiral duct is a complicated process that involves figuring out everything from the total length of ductwork needed to the volume of air needed to meet heating and cooling loads and may require consulting a professional.

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.

Things You Will Need

  • Tape measure
  • Pencil
  • Calculator
  • Energy-modeling software (optional)


  • This project requires technical knowledge and may require consulting a professional.

About the Author

William Vogel specializes in new art, old architecture and contemporary music. Originally from Chicago, he earned a B.A. in art history from Denison University in Granville, Ohio, as well as an M.S. in historic preservation from the Pratt Institute in Brooklyn, N.Y.