Ships classified as Panamax are of the maximum dimensions that will fit through the locks of the Panama Canal, each of which is 1000 ft long by 110 ft wide and 85 ft deep. Thus a Panamax ship will usually have dimension of close to 965 ft long (294m), 106 ft wide (32.3m) and a draft of 39.5 ft (12.04m). This is a tight fit that leaves little margin for piloting error.

This is considered a mid-size ship by today's standards. Many modern post-Panamax container ships are as large as possible, for greater cost-effectiveness. However, merchandise such as bulk grain products is moved primarily on Panamax ships.

U.S. Navy supercarriers are one type of vessel which are unable to fit through the canal locks.