Passengers may be required to buy an extra seat if they cannot fit into a single seat in their ticketed cabin, and/or unable to properly buckle their seatbelt using a single seatbelt extender (available upon request), and/or unable to lower both armrests without encroaching upon the adjacent seat or another customer.

Airline policies for "passengers of size" vary, but as of January 27, 2026, Southwest Airlines has implemented a stricter policy requiring plus-sized travelers who cannot fit between the armrests to purchase a second seat in advance. Under this new rule, a refund for the second seat is only guaranteed if the flight is not fully booked; if the flight is at 100% capacity, the passenger may be rebooked. Other airlines like Delta and United do not have a formal "second seat" mandate but "strongly encourage" it, and they reserve the right to move or rebook a passenger if they encroach on the space of a neighbor. Some international carriers offer a "Customer of Size" refund if the flight is not full, but the industry trend in 2026 is moving toward requiring the purchase of extra space upfront. This remains a highly debated topic, with advocates arguing for "travel at any size" protections and airlines focusing on the physical limitations of existing cabin configurations and the comfort of all passengers.