The 45 minute rule is the cutoff for checking bags and checking in on U.S. domestic flights. Passengers must check in and drop off any checked bags at least 45 minutes before scheduled departure. After the cutoff, airlines can mark the booking as a no-show and cancel the reservation.
The rule exists so ground crews have time to load checked bags into the cargo hold before the flight leaves.
Exceptions apply. Aspen (ASE), Honolulu (HNL), Las Vegas (LAS), and Miami (MIA) all enforce the 45 minute cutoff strictly. Many Caribbean and international airports require 60 minutes or more.
Carry-on-only travelers with a mobile boarding pass have more flexibility since no bags need to be loaded. Most airlines still recommend arriving 30 minutes before boarding closes.