United Airlines and American Airlines allow carry-on bags measuring a maximum of 14 inches in width, 22 inches in height and 9 inches in depth. Southwest Airlines allows carry-on bags measuring a maximum of 16 inches in width, 24 inches in height and 10 inches in depth, as of 2015.
In addition, United Airlines allows each passenger to carry a personal item on the plane, such as a laptop bag, backpack or shoulder bag. Personal items must measure no wider than 10 inches, no taller than 17 inches and no deeper than 9 inches. American Airlines also allows each passenger to bring along a single personal item on its flights but only specifies that the item must be small enough to fit under the seat in front of the passenger and must be smaller than the passenger’s carry-on bag.