Or you even forgot to add the number. Both things are not a problem, as all frequent flyer programs allow crediting miles retroactively. Depending on the program, you can credit the miles up to 12 months after the flight. Most programs, however, only allow this up to 6 months.

Yes, most major airlines allow you to retroactively claim miles or points for a flight you have already taken, provided you didn't wait too long. Typically, you can submit a request through the airline’s website or app between 3 days and 6 months (and sometimes up to 12 months) after the flight has completed. To do this, you will usually need your ticket number (a 13-digit code) and your boarding pass. Some airlines, like Delta and American, even allow you to claim credit for flights taken before you officially joined their loyalty program, though this is often limited to flights within the last 30 to 90 days. It is a "high-fidelity" travel habit to save your physical or digital boarding passes until the miles have officially posted to your account. If you forgot to add your number at check-in, don't worry—you aren't "out of luck," but you should file the request as soon as possible to ensure the data is still fresh in the airline's passenger manifest system.