As of 2015, the three major U.S. airlines have identical carry-on bag size restrictions, but discount airlines and international airlines have different limitations. Delta, American, and United all require that carry-on bags be smaller than 22 inches long by 14 inches wide by 9 inches tall.
Southwest Airlines and JetBlue allow for larger carry-on bags that measure up to 24 by 16 by 10 inches. Lufthansa requires that carry-on baggage be smaller than 22 by 16 by 8 inches. In addition to size restrictions, some airlines limit the weight of carry-on baggage.
Besides size, other restrictions apply to carry-on baggage due to Transportation Security Administration rules that forbid weapons, liquids and other potentially dangerous items. These rules apply to all flights within the United States, regardless of airline.