United Airlines check-in opens 24 hours before departure and closes 45 minutes before domestic flights and 60 minutes before international flights. Passengers can check in online via the website or mobile app using their confirmation number and last name. Online check-in allows seat selection, baggage check, and boarding pass download. Airport self-service kiosks open around 3 hours before departure and provide similar services. Curbside check-in is available at many U.S. airports starting 4 hours before the flight. Ticket counters also open about 3 hours before departure and close at the standard deadlines. United recommends arriving at the airport at least 2 hours before domestic flights and 3 hours before international flights. Boarding gates close 15 minutes before domestic flights and 30 minutes before international flights. Check-in cut-off times must be followed strictly. For support, passengers can call +1-866-551-1186.