Learn about the different methods for calculating a ship's tonnage.
Tonnage of a ship is not necessarily the number of tons that the ship weighs. There are at least six different methods of rating ships; the most common are:
Displacement tonnage -- used especially for warships and U.S. merchant ships -- is the weight of the water displaced by a ship. Since a ton of sea water occupies 35 cubic feet, the weight of water displaced by a ship can be determined by dividing the cubic footage of the submerged area of the ship by 35. The result is converted to long tons (2,240 pounds). Loaded displacement tonnage is the weight of the water displaced when a ship is carrying its normal load of fuel, cargo, and crew. Light displacement tonnage is the weight of water displaced by the unloaded ship.
Gross tonnage (GRST) or gross registered tonnage (GRT)-used to rate merchant shipping and passenger ships-is a measure of the enclosed capacity of vessel. It is the sum in cubic feet of the vessel's enclosed space divided by 100 (100 such cubic feet is considered one ton). The result is gross (registered) tonnage. For example, the old Queen Elizabeth did not weigh 83,673 tons, but had a capacity of 8,367,300 cubic feet.
Deadweight tonnage (DWT)-used for freighters and tankers-is the total weight in long tons (2,240 pounds) of everything a ship can carry when fully loaded. It represents the amount of cargo, stores, bunkers, and passengers that are required to bring a ship down to her loadline, i.e. the carrying capacity of a ship.
Net registered tonnage (NRT)-used in merchant shipping-is the gross registered tonnage minus the space that cannot be utilized for paying passengers or cargo (crew space, ballast, engine room, etc.)