Online check-in only asks for a specific, limited set of information — here's exactly what to have ready before you start, broken down by what's always required versus what only applies in certain situations.

Always Required

• Confirmation number (also called a record locator or booking reference) — typically a 6-character combination of letters and numbers, found in your booking confirmation email.
• Last name, exactly as it appears on the reservation.

These two pieces of information are enough to pull up most domestic reservations and begin the check-in process on nearly every airline.

Sometimes Required Instead

• Frequent flyer or loyalty program number — some airlines let you check in using this instead of a confirmation number, particularly if you're logged into your account.
• E-ticket number — occasionally requested as an alternative identifier, especially for bookings made through a travel agency rather than directly with the airline.

Required for International Flights

• Passport number.
• Passport expiration date.
• Country of citizenship.
• Destination address (where you'll be staying), required by some airlines and destination countries as part of Advance Passenger Information (API) requirements.

This information is typically collected once you begin the online check-in process for your flight, and it's worth having your passport physically on hand rather than trying to recall the details from memory.

Sometimes Requested, Depending on the Airline or Route

• TSA Redress Number — for travelers who have previously experienced watchlist-related screening issues and been issued a redress number to prevent future misidentification.
• Known Traveler Number (KTN) — for TSA PreCheck or Global Entry members, entering this can add expedited screening eligibility directly to your boarding pass.
• Visa information — for destinations requiring a visa, some airlines ask for visa details or confirmation during check-in as part of documentation verification.

Seat and Bag Preferences (Optional, Not Required Information)

• Seat selection — if you haven't already chosen a seat, check-in is often when you'll be prompted to pick or confirm one.
• Checked bag count — some airlines let you add and pay for checked bags during check-in rather than requiring this to be done separately beforehand.

What You Don't Need

• Your original booking confirmation email, if you already have the confirmation number — you don't need to have the email itself open or accessible, just the code.
• Payment information, unless you're adding a checked bag or paying for a seat upgrade during the process.
• A printed copy of anything in advance — the entire process, for most travelers, can be completed with just the confirmation number, last name, and (for international flights) passport details.

Where to Find Your Confirmation Number If You've Lost Track of It

If you can't locate your confirmation number, check the original booking confirmation email, or log into your account on the airline's site or app, where past and upcoming reservations are typically listed without needing the code directly.

A Quick Reference Checklist

• [ ] Confirmation number (or frequent flyer number/e-ticket number)
• [ ] Last name matching the reservation
• [ ] Passport details, if international
• [ ] TSA Redress Number or Known Traveler Number, if applicable
• [ ] Seat and bag preferences decided in advance (optional but helpful)

Bottom Line

For most domestic flights, online check-in requires nothing more than your confirmation number and last name. International flights add passport details to the mix, and travelers with TSA PreCheck, Global Entry, or a redress number can speed up security by entering those numbers during the same process. None of it requires advance paperwork beyond having your passport or loyalty numbers on hand.