The common dimensions of a 30-inch bag are 30 inches by 15 inches by 14 inches. Rolling bags usually have a height of 30 inches while duffel bags usually have a width of 30 inches.
If a 30-inch bag is measured for purposes of flight requirements or restrictions, the 30-inch measurement is the maximum measurement allowed for standard baggage rates. Most airlines require 30-inch bags to have no more than 62 inches in total dimensions comprised of the height plus width plus depth. Typically, 30-inch bags are considered to be luggage, and they are a common size to use when traveling.