The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures in long tons. Today, metric tonnes are more commonly used.
Ship displacement varies by a vessel's degree of load, from its empty weight as designed (known as "lightweight tonnage") to its maximum load. Numerous specific terms are used to describe varying levels of load and trim, detailed below.