What is deadweight tonnage (DWT)?

Deadweight tonnage (DWT) is the total weight a ship can safely carry, expressed in tonnes. It is the difference between the ship's loaded displacement, meaning its total weight when floating at the loaded draught, and its lightship weight, which is the empty ship of hull, machinery and permanent equipment. The figure usually quoted is the summer deadweight, measured at the summer load line, since the permitted draught, and therefore the deadweight, changes with load line zone and season.

DWT is not just cargo. It is everything the ship lifts: cargo plus fuel, fresh water, ballast, lubricating oil, stores, spares and crew. The usable cargo deadweight is therefore always less than the headline figure, and the difference matters commercially: on a long voyage needing heavy bunkers, cargo capacity shrinks accordingly. That is why deadweight is the standard measure of size for bulk carriers and tankers, whose trades are about weight lifted; the familiar size classes are defined by it, from Handysize bulkers through Panamax around 60,000 to 80,000 DWT, Capesize around 180,000 DWT, up to VLCC tankers in the region of 300,000 DWT.

Keep DWT distinct from two other measures that are often confused with it. Displacement is the actual weight of water the ship displaces at a given draught, so it equals lightship plus whatever deadweight is on board at that moment; warships are described by displacement, merchant ships rarely are. Gross tonnage and net tonnage are not weights at all: they are volume-derived figures calculated under the 1969 Tonnage Convention and used for regulatory thresholds, port and canal dues, manning and safety requirements. A car carrier can have a huge gross tonnage and a modest deadweight, because it is voluminous but carries light cargo.

In practice you meet deadweight everywhere: on the ship's certificates and stability information, in charter parties where cargo lift is being fixed, and in load line compliance, since loading beyond the marks is both illegal and unsafe. The working rule is simple: if the question is how much weight the ship can carry, the answer is deadweight; if it is how large the enclosed volume is, that is gross tonnage.