The passenger load factor (PLF) of an airline, sometimes simply called the load factor, is a measure of how much of an airline's passenger carrying capacity is used. It is passenger-kilometres flown as a percentage of seat-kilometres available.

This is a measure of capacity utilisation. As airlines frequently have heavy fixed costs and are capital intensive, the efficiency with which assets are used is crucially important.

This is an important efficiency measure, but it does not consider the pricing and the profitability at which the capacity is sold. It also implicitly assumes that the airline's fleet is fully utilised in terms of the number of kilometres flown.