Deadweight tonnage (DWT) is a measure for the total weight carrying capacity of a ship. Traditionally, this has been measured in long tons, where one long ton equals 2,240 pounds (1,016 kg). The tonne, or metric ton, of 1,000 kg (2,204.6 lb) is also now commonly used. Fuel, ballast, crew, drinking water, and crew provisions as well as cargo figure into the total.
Cargo deadweight tonnage is the weight of the ship's actual payload. Since the cargo portion of the weight is by far the most significant component, deadweight tonnage is often used to refer to a ship's total cargo capacity. The term may also sometimes be used to refer to a ship's actual cargo weight when carrying less than a maximum payload.
Measurement of a ship's cargo is sometimes a matter of weight, volume, or displacement. This is in turn tied to the safe handling of the vessel as well as fees charged for shipping, harbor use, and canal passage. Cargo capacity equates to earning capacity for a merchant vessel. This can be limited by the ship's maximum safe displacement and the volume of cargo space available.
If a ship were to take on 400 tons (about 363 metric tons) of cargo, it would also displace an equal amount of water, as would be indicated by a rise in the waterline. When the waterline rises to the Plimsoll line marking, the ship's maximum safe operating displacement has been reached. At this level of displacement the ship's deadweight tonnage has been reached, as well.
Modern maritime regulations have favored the use of volume measurements. A ship's gross tonnage (GT) is the volume of all its enclosed spaces. Its net tonnage (NT) is the volume of all cargo spaces. Profitable ship design would imply that filling the cargo space available with a typical payload mix would approach the maximum deadweight tonnage for safe operation.
Merchant ships are designed to enhance the cargo component of deadweight tonnage. A warship and a merchantman might have similar DWT. The merchant ship would be able to carry a greater cargo payload, however. The use of volume measurement for actual cargo space recognizes the disparity due to design that strict reliance on deadweight tonnage does not. It also promotes a uniform standard for enacting merchantman regulations, fees, and tariffs.
DWT is an adaptation from the early days of merchant sailing vessels. Wine was often shipped in casks, or tuns, holding 252 gallons (about 945 liters) and weighing about 2,240 pounds (1016 kg). The number of tuns aboard could be quickly estimated for taxation purposes by noting the displacement caused by the weight. This was indicated by reference to standard load lines painted on the hull of the ship.