Generally speaking, as stated by the policies of multiple airline companies drafted for plus-sized passengers, a person is considered ‘too big’ for an airplane seat if:
- Upon lowering the armrest, they are unable to fit in the space, therefore, encroaching upon the fellow passenger sitting next to them.
- If there is less than an inch of space between them (their body) and the person sitting next to them.
- If they are tight-squeezed into a seat with the armrests lowered and can not put on their seat belt.

Different airlines tackle this differently, but ultimately it boils down to this: if one seat in all its glory- the extension seatbelt as well as lack of armrest, does not fit you, then you will be expected to pay for the space that you take up and purchase two seats. Here are some of the airlines and their specific policies:

Allegiant expects those passengers who cannot lower the armrest to purchase two seats adjacent to one another ahead of time to ensure the availability of seats.

American Airlines, United Airlines, and Spirit Airlines specify that if a passenger’s body extends significantly outside the far end of the armrest, then an additional seat needs to be purchased. If you discover this after boarding but are unable to purchase adjacent seats, the flight allows the purchase of adjacent seats at a later flight without a price hike.

Southwest Airlines approaches the situation considerably and offers additional complementary seating (subject to availability) or offers to fully reimburse the cost of the second seat, which passengers are advised to book in advance to ensure availability.

Hawaiian Airlines, as well as Frontier Airlines, expect customers who cannot lower both their armrests to purchase two seats before travel.

Delta Airlines offer seatbelt extensions to passengers of size (but you cannot carry your own). They also recommend purchasing an additional seat or paying for an upgrade such that more seating room is available.

Outside the country, you’ll see that the policies are more accommodating. For instance, Air Canada recognizes obesity as a disability and offers extra seating free of charge within Canada to those who might need it. British Airways does not, however, seem to think so and mandates double seat booking, but Air France offers 25% off on the additional seat.