Cost per Available Seat Mile
This is calculated by dividing an airline's total operating expenses by its total ASMs.
This metric is the amount it costs an airline to fly one seat one mile. Cost per Available Seat Mile (CASM) is calculated per available seat mile, rather than per passenger mile, because the costs to an airline are essentially the same (minus the cost of beverage service and other small items) regardless of whether the seat is filled with a passenger or not.
CASM is the primary measure unit of cost in the airline industry- the lower the CASM, the more efficient the airline is operating and thus the more likely the airline will be profitable.