The speed of the air (FPM or m/s), which directly affects friction.
The diameter (round) or height and width (rectangular) of the duct. duct fitting loss calculator
Common tools for this include digital apps from industry leaders like Price Industries or online calculators. If you can tell me: The speed of the air (FPM or m/s),
By accurately sizing fans, you prevent over-pressurizing the system, which wastes energy. you prevent over-pressurizing the system