It would be great if airlines had consistent rules for size and weight of carry-on bags, but not all overhead baggage storage is created equal. Carriers base their limits on the internal configurations of the planes in their fleet. This means that bags that can fly with you on one carrier may have to be checked on another.
Legacy domestic carriers like American, Delta, United, and US Airways agree on a maximum size of 22 by 14 by 9 inches. Most carry-ons push those dimensions to the limit, and many bags sold as carry-ons are too big when you factor in handles and wheels.
A few domestic airlines are more generous with carry-on allowances: Alaska Airlines permits them up to 24 by 17 by10 inches, while AirTran and Southwest permit 24 by 16 by 10 inches. Domestic airlines rarely care about the weight of carry-ons.
European carriers are usually more stingy about in-cabin space, and tend to limit the weight of bags you bring aboard. Travelers who fly from this country to Europe on a North American airline are often unhappy to discover that they have to check their carry-on bags when they continue their journey on a European carrier. The new, less-padded seats many airlines have installed to squeeze in more rows of passengers place all the support structure as well as the life jacket beneath the seat cushion. This often means less than eight inches of clearance for bags under the seat in front of you, and a maximum width of 12 to 14 inches. The dreaded middle seat in a three-seat row has slightly more space under the seat.
Overhead requirements are also often more stringent. Air France and Alitalia limit bags to 21 by 13 by 9 inches with an allowable maximum weight of 26.4 pounds. Carry-on baggage on KLM, Swiss, Lufthansa, and Turkish can be two inches wider at 21 by 15 by 9 inches but cannot weigh more than 17.6 pounds.
The lesson is that frequent overseas travelers find it worth purchasing an ultra-light version of the smaller 21-by-13-by-9-inch carry-on or roll-aboard. That size will fly almost anywhere if you don’t load it down with heavy contents.
Overhead bins are unforgiving, so measure before you schlep a bag all the way to the door of the plane, only to be forced to surrender it.