The amount of surface air entering a fan through the casing at the top of the upcast shaft, the airlock doors, and the fan-drift walls. The extent of leakage will depend on the fandrift water-gage method of construction, the number and type of entrances, and whether the upcast shaft is used for winding. The surface leakage at airlocks may vary from about 25,000 ft ₃/min (708 m₃/min) at 5-in (12.7-cm) water gage to about 55,000 ft₃/min (1,557 m₃/min) at 25-in (63.5-cm) fan-drift water gage.