Maybe, maybe not.
Ductulators and duct sizing programs are all based on an equivalent round duct size. The airflow through a duct is limited by both velocity and pressure loss. With a specific flow rate, in cubic feet per minute (CFM), and a round (or rectangular) duct size, both the velocity, in feet per minute (FPM), and friction loss, in inches of water per 100 ft of duct, can be directly read off of the ductulator. Friction loss in a duct is a function of the roughness of the inside surface of the duct and the perimeter of the duct. The higher the velocity of the air traveling through the duct, the higher the friction loss. The rougher the surface, the higher the friction loss. What is not so obvious is the effect that perimeter has on the friction loss.