Exhaust Fan & Door GapPick the room and the standard. Get the airflow the fan must pull in L/s, m³/h and cfm, the fan or duct size, the air changes it gives, and the gap under the door the replacement air needs to get in.

Air changes = 0 uses the code rate alone; enter e.g. 10 to size by air changes too (the larger wins). A small wall fan works at about 10 to 25 Pa: the lower the suction, the bigger the gap must be. Use a transfer grille if the gap gets too big.