Ships' tonnage is a measure of capacity (volume) not weight (mass) and dates back to ancient times when a ship‟s capacity was calculated by the number of tuns (barrels) that it could hold. Gross tonnage, compensated gross tonnage, and net tonnage should therefore be expressed in tons. However, for the actual weight of goods carried by a vessel, the lifting capacity of its cranes, or the steel used in a ship's construction will use tonnes.
Compensated gross tonnage (cgt) A measure (in tons) of the output of a shipyard that takes into account the different levels of work needed to build ships of different types. The ordinary gross tonnage figure is modified accordingly.
Deadweight (dwt) Weight in tonnes of cargo, stores, fuel, passengers and crew carried by a ship when loaded to the maximum summer loadline.
Displacement tonnage Amount of water displaced by the vessel (which will vary depending on whether the vessel is loaded or empty).
Gross registered tonnage (grt) Capacity, in cubic feet, of the spaces within the hull and of the spaces above the deck available for cargo, stores, passengers and crew (with certain exceptions). The figure is divided by 100, hence 1ft³ of capacity = 1 gross ton. This measurement is no longer in common use.
Gross tonnage (gt) The moulded volume of all enclosed spaces of the vessel. A formula is then applied to those measurements, hence no unit of measurement is assigned and the figure is simply referred to as the vessel's “gross tonnage”.
Net tonnage (nt) The volume of a ship‟s payload spaces, taking account of depth, draught, and number of passengers, but never exceeding gt × 0.3. No unit of measurement is assigned and the figure is simply referred to as the vessel's “net tonnage”.