In 1948, research by R.G. Huebscher established flow resistance for different shapes of ductwork at equal velocities, which allowed the creation of the duct-sizing calculator or “Ductulator” for picking equivalent sizes of rectangular ducts based on the baseline round duct size.
The Ductulator is a shortcut that only works on paper if you reduce the velocity pressure to a negligible factor around 1,500 feet per minute or less in any ductwork system. At the maximum 1,500 fpm, the velocity pressure is 0.14 inches water column and can be dealt with using balancing dampers. The duct calculator works great for low-velocity designs commonly found in residential and light-commercial work.