Carry-on baggage limits at the largest American airlines.

Allegiant Air
Included baggage: One personal item (40×35×17 cm) under the seat.
Extra option: Carry-on bag (max. 56×35×23 cm) available for purchase.
Weight limit: No official limit, but it must be liftable into the overhead bin.
Control: Strict with dimensions – extra fees apply if exceeded.

United Airlines
Included baggage: One personal item (43×25×22 cm) – only for “Basic Economy”.
Extra option: Standard tickets include a carry-on bag (56×35×23 cm).
Weight limit: Not officially stated, but must fit safely in the cabin.
Control: Careful gate checks for the cheapest fares.

Frontier Airlines
Included baggage: One personal item (40×35×20 cm) under the seat.
Extra option: Carry-on bag (max. 56×36×23 cm) must be purchased.
Weight limit: Up to 15 kg for paid cabin luggage.
Control: Very strict – extra fees for non-compliance.

Spirit Airlines
Included baggage: One personal item (45×35×20 cm) under the seat.
Extra option: Carry-on bag (max. 56×46×25 cm) available for an extra fee.
Weight limit: Up to 18 kg for cabin luggage.
Control: Known for strict enforcement – size and weight are checked often.

JetBlue Airways
Included baggage (Blue Basic): One personal item (43×33×20 cm).
Extra option: Standard fares include a carry-on bag (56×35×23 cm).
Weight limit: No official limit, but bag must be easy to handle.
Control: Blue Basic fares do not include carry-on unless paid for.

Delta Air Lines
Included baggage: One personal item and one carry-on bag (56×35×23 cm).
Weight limit: Bags must fit safely – no strict weight enforcement.
Extra option: No fee for carry-on, even with Basic Economy fares.
Control: Generally relaxed, though checks may happen on full flights.

Conclusion
Ultra low-cost U.S. airlines like Spirit, Frontier, and Allegiant offer very cheap tickets, but almost all baggage must be purchased separately. Delta, JetBlue, and United offer more flexibility – but only with certain fare types. Always check your ticket conditions carefully – and measure your bags, especially with basic fares.