Deadweight Tonnage

Deadweight tonnage is the single most important figure for describing the size of cargo ships such as bulk carriers and tankers, because those trades care about how much weight the vessel can lift. It is calculated as the loaded displacement, the total weight of water the ship displaces when floating at its maximum permitted draught — minus the lightship weight, which is the empty ship comprising hull, machinery and permanent equipment. The result is the total weight the ship can take on before it sits down to its load line.

A common mistake is to treat deadweight as cargo capacity. It is not. Deadweight includes everything the ship loads: cargo, but also bunker fuel, fresh water, ballast water, lubricating oils, provisions, spare parts and the crew and their effects. The usable cargo deadweight is therefore always somewhat less than the headline DWT, and it falls as more fuel and stores are carried for a long voyage. Charterers pay close attention to the cargo deadweight available on the specific voyage, not just the vessel's nominal figure.

Deadweight should not be confused with the two other tonnage measures that appear on a ship's certificates. Displacement is the actual weight of the ship and everything in it at a given moment. Gross tonnage and net tonnage are volume-based figures, not weights, used for regulations, manning, safety requirements and the calculation of port and canal dues. So when a vessel is described as, for example, a 180,000 DWT capesize bulk carrier, that number is a statement of lifting capacity in tonnes, and it is the figure brokers and operators use first when matching a ship to a cargo.