The May 2013 system load factor of 84.2 percent, the domestic load factor of 84.8 percent and the international load factor of 83.0 percent were all record highs for the month of May as year-over-year growth in revenue passenger-miles exceeded both domestic and international capacity expansion. Load factor is a measure of the use of aircraft capacity that compares Revenue Passenger-Miles (RPMs) as a proportion of Available Seat-Miles (ASMs). 

In May, Delta Air Lines carried more system passengers than any other U.S. airline.  Southwest Airlines carried the most domestic passengers. United Airlines carried the most international passengers. The top 10 U.S. airlines in terms of number of passengers carried 80.4 percent of systemwide passengers, up from 79.9 percent carried by the U.S. airlines that were in the top 10 in May 2012. 

During the first five months of 2013, Delta carried more system passengers than any other U.S. airline and Southwest carried the most domestic passengers.  United carried the most international passengers. The top 10 U.S. airlines carried 80.6 percent of systemwide passengers, up from 79.8 percent carried by the U.S. airlines that were in the top 10 during the first five months of 2012. In May, more total system and domestic passengers boarded planes at Atlanta Hartsfield-Jackson International than at any other U.S. airport and more international passenger’s boarded U.S. carriers at Miami than at any other U.S. airport.