The passenger load factor (PLF) of an airline, sometimes referred to as the seat load factor (SLF) or simply load factor, is a measure of how much of the airline seat carrying capacity is used.
The definition is:
passenger-kilometres flown divided by the seat-kilometres available (expressed as a percentage).
In effect, this term is an assessment of an airline’s capacity utilization.
Many airlines will have a PLF in the region of 70-80 percent.
A PLF of less than 50 percent means that over half the seats are empty – a sign the airline is in big trouble considering that airlines incur huge fixed costs from having to buy or rent their airplanes.