The diameter (round) or height and width (rectangular) of the duct.
The speed of the air (FPM or m/s), which directly affects friction. duct fitting loss calculator
Proper sizing prevents noisy systems and ensures adequate air delivery to rooms. The diameter (round) or height and width (rectangular)
Specifically identifying if it's a 90-degree elbow, a tee, or a reducing transition. Why You Should Use One you prevent over-pressurizing the system
By accurately sizing fans, you prevent over-pressurizing the system, which wastes energy.
I can help walk you through the calculation or direct you to a specific tool. Duct Pressure Loss Calculator | Plastec Ventilation
The volume of air (CFM or L/s) passing through the fitting.