Most airlines close the gate 15–20 minutes before departure for domestic flights and 30–45 minutes for international. The time on your boarding pass is the boarding start, not the deadline.
Airlines publish a gate-closing time in their conditions of carriage and print a boarding time on the pass. The boarding time is when the first groups are called; the gate typically closes 15 minutes before departure on US domestic flights, 20–30 on European flights and up to 45 on long-haul.
If you arrive after the gate closes, you are a no-show even if the aircraft is still at the stand. Airlines rarely reopen the door.
Practical rule: be at the gate when boarding begins, not when it ends. Long walks, bus gates and passport checks at the gate (common for US-bound flights) all eat the margin.
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 →
What happens if I miss my connecting flight?
On a single ticket, the airline rebooks you on the next available flight at no charge, and may owe meals or a hotel if the delay was its fault. On separate tickets you're on your own — you'll buy a new ticket. 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 Request a correction if something is out of date.