The three-letter codes on your ticket and bag tag (JFK, LHR, SIN) are IATA codes used by airlines. The four-letter codes (KJFK, EGLL, WSSS) are ICAO codes used by pilots and air traffic control. Some codes are historic — ORD comes from Orchard Field.
IATA codes are assigned by the International Air Transport Association and are what travelers see. They often match the city (LIS, MAD) or the airport name (CDG for Charles de Gaulle), and sometimes history (ORD for Chicago’s old Orchard Field, YYZ for Toronto’s original rail-station code).
ICAO codes are four letters, with the first letter(s) indicating the region: K for the contiguous US, EG for the UK, LF for France, RJ for Japan, WS for Singapore.
City codes cover multiple airports: NYC (JFK, EWR, LGA), LON (LHR, LGW, STN, LCY, LTN), TYO (HND, NRT), PAR (CDG, ORY). Searching a city code shows all of them.
Every airport guide on this site lists both codes at the top.
Related questions
How early should I get to the airport?
Two hours before a domestic flight, three before an international one, and add 30–45 minutes at mega-hubs, on holiday weekends, or if you're checking bags. PreCheck/Fast Track lets you trim about 30 minutes. Full answer →
Airline, airport and government rules change without notice. Treat this as a starting point and confirm with the airline or authority for your specific trip. Request a correction if something is out of date.