How are flight numbers assigned?

Numbering conventions vary widely from airline to airline. A few general guidelines apply to most carriers: One- and two-digit flight numbers are generally reserved for long-haul or other premium flights. Flight number 1 is often for an airline’s “flagship” service. Four-digit numbers under 5000 are typically used for regional affiliate flights and over 5000 are often, though not always, codeshare flights. Beyond that, practices are individual by airline. Some carriers assign odd or even numbers according to the direction of the flight. Other carriers will use consecutive numbers to indicate a pair of flights, such as 222 on the outbound and 223 on the reverse inbound flight. Where there are multiple flights a day on the same route the flight numbers tend to increase throughout the day. Flight numbers are usually retired after a crash or serious accident.