Check-in deadlines are much stricter than many travelers realize and are usually calculated based on the flight's departure time, not the boarding time. For most domestic U.S. flights, the "hard" cutoff for checking in—whether online or at the counter—is typically 45 minutes before departure if you have bags to check, and 30 to 45 minutes for carry-on only. For international travel, this window usually closes 60 to 75 minutes before takeoff. It is important to note that "boarding" usually begins 30 to 50 minutes before departure, meaning that if you arrive at the airport 5 minutes before boarding starts, you may already be locked out of the system. Some low-cost carriers (like Ryanair or Spirit) have even more aggressive cutoffs to maintain their tight schedules. Furthermore, if you are not at the boarding gate at least 15 minutes before the scheduled departure (10 minutes for some airlines), the gate agent has the legal right to give your seat to a standby passenger and close the flight, even if you have already checked in and are standing in the security line.