ducts are named after their diameter (8-inch duct means that diameter is 8 inches). On top of that, we are talking inches here, but both FPM and CFM are in feet, not inches. That’s why we need to include 12 inches = 1 foot conversion here as well, and we get this formula: ...
FPM refers to the "air speed" produced by a dryer, as measured in Feet Per Minute. CFM is a measure of the "air flow," stated in Cubic Feet per Minute. In