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.
The process of determining a vessel's displacement begins with measuring its draft This is accomplished by means of its "draft marks" (or "load lines"). A merchant vessel has three matching sets: one mark each on the port and starboard sides forward, midships, and astern. These marks allow a ship's displacement to be determined to an accuracy of 0.5%.
To calculate the weight of the displaced water, it is necessary to know its density. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³); so a ship will ride higher in salt water than in fresh. The density of water also varies with temperature.