A common question, and the guidance below is more relevant to domestic type installations.

Domestic Axial fans

An axial fan has a fan blade like a ship propeller. These are low pressure fans and a general rule of thumb for these is to use no more than 1.5 metres of ducting. On a 100mm fan/ducting system, a 90 degree bend is equivalent to 1 metre, preferably rigid duct. For longer runs, a centrifugal type fan should be considered.

Domestic Centrifugal fans

A Centrifugal fan has a fan blade like a hamster wheel. The question of duct run length becomes more difficult here. Centrifugal fans are higher pressure fans and so cope better with ducted runs or where installations are in exposed positions such as an external wall prone to strong gusty winds. To know how far a Centrifugal fan is going to push the air means knowing the overall system resistance/pressure - Anything in the path of the airflow causes a resistance - this could be ducting straight lengths, bends, filters, grilles etc..... In a domestic wet room, you need to be considering a Centrifugal fan once you are over 1.5 meters of ducting, but you must still ensure that the chosen fan will achieve the required airflow for the room being ventilated with the system resistance taken into account.