Passengers who cannot lower both armrests or whose torso extends into another passenger's seating space must purchase a ticket for a second seat. If no additional seats are available, you may not be able to fly.

Most airlines, including Southwest, Delta, and United, do not have a specific weight or height requirement for buying a second seat; instead, they use the "armrest test." If you cannot comfortably sit in a single seat with the armrests lowered, or if you "encroach" upon the space of the neighboring passenger, you are typically required to purchase an additional seat. On Southwest Airlines, the "Customer of Size" policy allows passengers who exceed the seat width to purchase a second seat and, as of early 2026, still request a refund after travel (though rules have become stricter regarding fare classes and flight capacity). For most other carriers, if you anticipate needing extra room, you must "proactively" buy the second seat at the time of booking to ensure it is adjacent to you. Failing to do so can result in being denied boarding if the flight is full and you cannot be safely accommodated in a single seat. It is always best to check the specific "Passenger of Size" policy for your airline, as dimensions for seats vary by aircraft type.