Definition: Gross tons
Formerly known as "gross registered tons", this is the fundamental measurement of the physical size of a ship. It refers to the volume enclosed by the ship’s hull and superstructures in hundreds of cubic feet. All registered ships will be assessed for their gross tonnage and this is the parameter normally referred to when the size of a merchant ship is quoted in tons (The use of the word ton in this case refers to the old English word "tun", meaning a barrel. The designation of "tunnage" with reference to a ship was originally a measure of the capacity of a ship in terms of the number of barrels it can carry, hence its relation to volume rather than weight. The correct unit in this case is tons and not the metric tonnes).
First Marine International Limited, "Overview of the international commercial shipbuilding industry, Background report", disseminated on the Web site of Directorate-General "Enterprise and Industry" of the European Commission, May 2003