The FAA standard for a maximum-sized carry on is 45 linear inches. This figure accounts for the sum of the length, height and width of the bag. This allows for many different configurations of carry on luggage, from rolling bags to duffels and backpacks. The FAA number is a general ceiling, however. Most airlines place maximums on length, height and width that are much more defined. For example, American Airlines' maximums include 22 inches in length, 14 inches in width, and 9 inches in height which equals 45 inches. Bags that exceed these limits will be required to be checked, even if they nevertheless meet the 45 inch requirement.
Before arriving at the airport with your carry on bag, check with the airline to see if it will fit in the overhead compartment on the aircraft scheduled for your flight. The smaller the plane, the less overhead compartment space. Airlines commonly limit the size of the carry-on to sizes far under the FAA maximums for particularly small aircraft and sold-out flights. Contact the airline on their toll free customer service number, and give them information on what flight you are on. A customer service representative should be able to give you acceptable dimensions for carry-on luggage specific to your flight.
Airlines generally offer a sizing box at the luggage check in. This box is specially designed with the airline's precise length, width, and height requirements in mind. Bags that meet the requirements will fit, but those that don't won't -- even if they would otherwise fit into the overhead compartment. The carry-on should fit easily into the sizing box without being forced. Luggage that does not easily fit must be checked to avoid on-board problems or conflicts.
In addition to dimension restrictions, many airlines also regulate the weight of bags that can be placed into the overhead compartments. For most airlines, the maximum weight is forty pounds. The bag should be easy for you to lift over your head and place in the overhead compartment without struggle. There are other passengers on the plane and you do not want to accidentally drop your heavy luggage on another passenger. Overweight baggage can also impair the structural integrity of the compartment.
