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Tonnage is a measure of the size or cargo carrying capacity of a ship . The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Tonnage should not be confused with Displacement which refers to the loaded or empty weight of the vessel itself.

Measurement of tonnage can be less than straightforward, not least because it is used to assess fees on commercial shipping .

Tonnage measurements

Gross tonnage ( GT ) is a function of the volume of all ship's enclosed spaces (from keel to funnel ) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.8316  m ³ ; 200 GT = 274 GRT = 775,88 m ³ ; 500 GT = 665 GRT = 1,883.07 m ³ ; 3,000 GT = 3,776 GRT = 10,692.44 m ³ ), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity.

Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.

It is calculated by using the formula : , where V = total volume in m ³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : ), so that, for a ship of 10,000 m ³ total volume, the gross tonnage would be 0.28 × 10,000 = 2,800. GT is consequently a measure of the overall size of the ship. For a ship of 80,000 m ³ total volume the gross tonnage would be 0.2980617 × 80,000 = 23,844.94 GT.

Net tonnage (NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship.

A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc., are based on its gross tonnage, GT, or net tonnage, NT.

Gross register tonnage ( GRT ) represents the total internal volume of a vessel, where a register ton is equal to a volume of 100 cubic feet (2.83168 m ³ ), which volume, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonnes . The definition (and calculation) of the internal volume is complex; a ship's hold can, for instance, be assessed for bulk grain (accounting for all the air space in the hold) or for bales (omitting the spaces into which bulk, but not baled cargo would spill). If V stands for the total internal volume in m ³ , then the GRT equals V / 2.83168, so for a ship of 10,000 m ³ total internal volume, the gross register tonnage is 10,000 / 2.83168 = 3531.47 GRT. Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, and is no longer a widely used term in the industry. ^{[1] [2]}

Net register tonnage ( NRT ) is the volume of cargo the vessel can carry; i.e., the gross register tonnage less the volume of spaces that will not hold cargo (e.g., engine compartment, helm station, crew spaces, etc., again with differences depending on which port or country is doing the calculations). It represents the volume of the ship available for transporting freight or passengers . It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

The Panama Canal/Universal Measurement System (PC/UMS) is based on net tonnage , modified for Panama Canal purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; a PC/UMS net ton is equivalent to 100 cubic feet of capacity. ^[3]

The Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the Moorsom System and was established by the International Commission of Constantinople in its Protocol of 18 December 1873. It is still in use, as amended by the Rules of Navigation of the Suez Canal Authority, and is registered in the Suez Canal Tonnage Certificate .

Thames measurement tonnage is another volumetric system, generally used for small vessels such as yachts ; it uses a formula based on the vessel's length and beam.

Weight measurements

While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:

Lightship or Lightweight measures the actual weight of the ship with no fuel , passengers, cargo , water, etc. on board.

Deadweight tonnage (often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel , water, and stores. Like Displacement, it is often expressed in long tons or in metric tons.

Metric Tonnes per Centimetre Immersion (usually abbreviated to TPC or TPCMI ) is the number of Metric Tonnes (1,000kg) that need to be loaded on the ship for the salt water draft to increase by one centimetre.

Imperial Tons per Inch immersion (usually abbreviated to TPI ) is the number of Imperial Long Tons (2,240 lbs) that need to be loaded on a vessel for the draft to increase by one inch. Old imperial TPI measurements are still occasionally used within the USA and the Panama canal. As no ship has been measured by a Classification Society since the 1950s using Imperial measures, modern TPI figures are therefore a conversion from the original Metric measurements and should not be relied upon to be accurate.

The TPCMI figure is used to calculate the draft of the vessel with a given DWT of cargo loaded. On a typical Panamax Bulk carrier with a TPCMI of 80, it would mean that the ship will sink (Draft increase) by one centimetre for every 80 tonnes of cargo loaded.

Draft The distance, usually measured in meters, between the lowest point of the keel and the waterline which varies dependant on the load ( DWT ) the vessel has on board.