Revenue passenger miles (RPMs) and Revenue passenger kilometers (RPKs) are measures of traffic for an airline

Airline

An airline provides air transport services for traveling passengers and freight. Airlines lease or own their aircraft with which to supply these services and may form partnerships or alliances with other airlines for mutual benefit...

 flight, bus

Bus

A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...

, or train

Train

A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

 calculated by multiplying the number of revenue-paying passengers

Revenue passenger

A revenue passenger is someone who has paid the transport operator for her or his trip. That excludes non-paying passengers such as airline employees flying on free or nearly-free passes, babies and children who do not have a seat of their own, etc...

aboard the vehicle by the distance traveled. On long-distance buses and trains (and some planes) passengers may board and disembark at intermediate stops, in which case RPMs/RPKs have to be calculated for each segment if a careful total is needed.