Last updated

Tonnage is a measure of the cargo-carrying capacity of a ship, and is commonly used to assess fees on commercial shipping. The term derives from the taxation paid on tuns or casks of wine. In modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Although tonnage (volume) should not be confused with displacement (the actual weight of the vessel), the Imperial ton of 2240 lbs is derived from the fact that a "tun" of wine typically weighed that much.


Tonnage measurements

Tonnage measurements are governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which initially applied to all ships built after July 1982, and to older ships from July 1994. [1]

Gross tonnage (GT) is a function of the volume of all of a 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 of gross register tonnage (GRT). Gross tonnage is therefore 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 it derives from the volumetric capacity in cubic metres.

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, and the like may be based on its gross tonnage (GT) or net tonnage (NT).

Gross register tonnage (GRT) represents the total internal volume of a vessel, where one register ton is equal to a volume of 100 cubic feet (2.83  m3 ); a volume that, if filled with fresh water, would weigh around 2.83 tonnes. The definition and calculation of the internal volume is complex; for instance, a ship's hold may 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). Gross register tonnage was replaced by gross tonnage in 1982 under the Tonnage Measurement convention of 1969, with all ships measured in GRT either scrapped or re-measured in GT by 1994. [2] [1]

Net register tonnage (NRT) is the volume of cargo the vessel can carry—that is, the gross register tonnage less the volume of spaces that do not hold cargo (e.g., engine compartment, helm station, and crew spaces, again with differences depending on which port or country does 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; one PC/UMS net ton is equivalent to 100 cubic feet (2.83 m3) of capacity. [3]

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 (TM) 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,[ citation needed ] referred to as such:

Lightship or lightweight measures the actual weight of the ship with no fuel, passengers, cargo, water, and the like 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 (tonne).[ citation needed ]

Metric tonnes per centimetre immersion (usually abbreviated to TPC or TPCMI) is the number of metric tonnes (1,000 kg) that need to be loaded on the ship for the salt water draft (draught) to increase by one centimetre. The TPCMI is used to calculate the draft of the vessel with a given deadweight tonnage of cargo loaded. For a typical Panamax bulk carrier with a TPCMI of 80, the ship will sink (i.e., its draft will increase) by one centimetre for every 80 tonnes of cargo loaded.[ citation needed ]

Imperial tons per inch immersion (usually abbreviated to TPI) is the number of imperial long tons (2,240 lb) that need to be loaded on a vessel for the draft to increase by one inch. Old imperial TPI measurements are still[ when? ] occasionally used within the United States 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.[ original research? ]

Non-maritime usage of the term tonnage

Tonnage can refer to the quantity of a mineral or the mineral ore extracted from a mine. It may refer to the production of any commodity that is normally expressed in tons or tonnes. The term can also apply to the total weight drawn by a railway locomotive, or the total weight of freight passing over a railway line or road.

The tonnage may be expressed as 1 tonne (1,000 kg; 2,205 lb), 1 short ton (907.2 kg; 2,000 lb) or 1 long ton (1,016 kg; 2,240 lb). Often this distinction is not of any importance, however sometimes it is critical to define the exact units in which the tonnage is expressed.


Historically, tonnage was the tax on tuns (casks) of wine [ by whom? ] that held 954 litres (252 gallons) of wine and weighed 1016 kilograms (2,240 pounds). This suggests that the unit of weight measurement, the long ton (1,016 kg or 2,240 lb), and tonnage share the same etymology. The confusion between weight-based terms (deadweight and displacement) stems from this common source and the eventual decision to assess dues based on a ship's deadweight rather than counting the tuns of wine. In 1720 the Builder's Old Measurement Rule was adopted[ by whom? ] to estimate deadweight from the length of keel and maximum breadth or beam of a ship. This overly simplistic system was replaced by the Moorsom System in 1854 and calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.

When steamships came into being, they could carry less cargo, size for size, than could sailing ships. In addition to space taken up by boilers and steam engines, steamships carried extra fresh water for the boilers and coal for the engines. Thus, to move the same volume of cargo as a sailing ship, a steamship would be considerably larger than a sailing ship.[ citation needed ]

Harbour dues are based on tonnage. In order to prevent steamships operating at a disadvantage, various tonnage calculations were established to minimize the disadvantage presented by the extra space requirements of steamships. Rather than charging by length, displacement, or the like, charges were calculated based on the viable cargo space. As commercial cargo sailing ships are now largely extinct, gross tonnage is becoming the universal method of calculating ships' dues, and is also a more straightforward and transparent method of assessment.

See also


  1. 1 2 International Convention on Tonnage Measurement of Ships, 1969 Archived 2008-01-16 at the Wayback Machine , International Maritime Organisation. Retrieved May 10, 2006.
  2. CWP Handbook of Fishery Statistical Standards. Retrieved May 10, 2006.
  3. Panama Canal Tolls, from the Panama Canal Authority. Retrieved May 10, 2006.

Related Research Articles

Ton Unit of mass or volume with different values

The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of mass. Its original use as a measurement of volume has continued in the capacity of cargo ships and in terms such as the freight ton. Recent specialised uses include the ton as a measure of energy and for truck classification. It is also a colloquial term.

<i>Seawise Giant</i>

TTSeawise Giant—earlier Oppama; later Happy Giant, Jahre Viking, Knock Nevis, and Mont—was a ULCC supertanker that was the longest self propelled ship ever, built by Sumitomo Heavy Industries in Yokosuka, Kanagawa, Japan. It possessed the greatest deadweight tonnage ever recorded. Fully laden, its displacement was 657,019 tonnes.

Passenger ship Watercraft intended to carry people onboard

A passenger ship is a merchant ship whose primary function is to carry passengers on the sea. The category does not include cargo vessels which have accommodations for limited numbers of passengers, such as the ubiquitous twelve-passenger freighters once common on the seas in which the transport of passengers is secondary to the carriage of freight. The type does however include many classes of ships designed to transport substantial numbers of passengers as well as freight. Indeed, until recently virtually all ocean liners were able to transport mail, package freight and express, and other cargo in addition to passenger luggage, and were equipped with cargo holds and derricks, kingposts, or other cargo-handling gear for that purpose. Only in more recent ocean liners and in virtually all cruise ships has this cargo capacity been eliminated.

Twenty-foot equivalent unit Unit of cargo capacity, TEU

The twenty-foot equivalent unit is an inexact unit of cargo capacity, often used for container ships and container ports. It is based on the volume of a 20-foot-long (6.1 m) intermodal container, a standard-sized metal box which can be easily transferred between different modes of transportation, such as ships, trains, and trucks.

Turret deck ship Ship type with hull topsides rounded and stepped inwards to a narrow weather deck

A turret deck ship is a type of merchant ship with an unusual hull, designed and built in the late 19th and early 20th centuries. The hulls of turret deck vessels were rounded and stepped inward above their waterlines. This gave some advantages in strength and allowed them to pay lower canal tolls under tonnage measurement rules then in effect. The type ceased to be built after those rules changed.

Deadweight tonnage Unit of mass

Deadweight tonnage or tons deadweight (DWT) is a measure of how much weight a ship can carry. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew.

Gross register tonnage or gross registered tonnage, is a ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m3). Gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume. Typically this is used for dockage fees, canal transit fees, and similar purposes where it is appropriate to charge based on the size of the entire vessel.

Gross tonnage

Gross tonnage is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage. Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weight such as deadweight tonnage or displacement.

Builder's Old Measurement is the method used in England from approximately 1650 to 1849 for calculating the cargo capacity of a ship. It is a volumetric measurement of cubic capacity. It estimated the tonnage of a ship based on length and maximum beam. It is expressed in "tons burden", and abbreviated "tons bm".

The Moorsom System is a method created in the United Kingdom of calculating the tonnage or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees. It was put into use starting in 1849 and became British law in 1854.

Displacement (ship) Ships weight

The displacement or displacement tonnage of a ship is its weight. As the term indicates, it is measured indirectly, using Archimedes' principle, by first calculating the volume of water displaced by the ship, then converting that value into weight. Traditionally, various measurement rules have been in use, giving various measures in long tons. Today, metric tonnes are more commonly used.

Net tonnage Dimensionless index calculated from the total moulded volume of the ships cargo spaces by using a mathematical formula

Net tonnage is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula. Defined in The International Convention on Tonnage Measurement of Ships that was adopted by the International Maritime Organization in 1969, the net tonnage replaced the earlier net register tonnage (NRT) which denoted the volume of the ship's revenue-earning spaces in "register tons", units of volume equal to 100 cubic feet (2.83 m3). Net tonnage is used to calculate the port duties and should not be taken as less than 30 per cent of the ship's gross tonnage.

Net register tonnage is a ship's cargo volume capacity expressed in "register tons", one of which equals to a volume of 100 cubic feet (2.83 m3). It is calculated by subtracting non-revenue-earning spaces i.e. spaces not available for carrying cargo, for example engine rooms, fuel tanks and crew quarters, from the ship's gross register tonnage. Net tonnage is thus used in situations where a vessel's earning capacity is important, rather than its mere size. Net register tonnage is not a measure of the weight of the ship or its cargo, and should not be confused with terms such as deadweight tonnage or displacement.

<i>Clementine Maersk</i>

Clementine Maersk is a container ship of the Maersk Line. The ship was built in 2002 in the shipyard of Odense Steel and has a capacity of 6,600 TEUs according to company statistics and calculations.

Ship measurements consist of a multitude of terms and definitions specifically related to ships and measuring or defining their characteristics.

USS <i>West Hobomac</i> (ID-3335)

USS West Hobomac (ID-3335) was a steel–hulled cargo ship which saw service with the U.S. Navy as an auxiliary during World War I, and which later operated under the British flag during World War II before being lost to enemy action.

<i>Pierre Guillaumat</i> (supertanker)

Pierre Guillaumat was a supertanker built in 1977 by Chantiers de l'Atlantique at Saint-Nazaire for Compagnie Nationale de Navigation. It was the third vessel of Batillus class supertankers and distinguished for being the biggest ship ever constructed. It was surpassed in length, deadweight tonnage and displacement only by Seawise Giant, which, though it was originally smaller when it was built in 1976, was subsequently lengthened and enlarged.

<i>Thunder Bay</i> (ship)

Thunder Bay is a Trillium-class lake freighter cargo vessel, built and launched in China in 2013. The ship is owned, and operated on the Great Lakes, by the Canada Steamship Lines (CSL). Like her three sister ships in CSL's Trillium class, Baie St. Paul, Baie Comeau, and Whitefish Bay, the vessel is a self-unloading bulk carrier, with a conveyor belt on a long boom that can be deployed over port or starboard sides.

<i>Baie Comeau</i> (2013 ship)

Baie Comeau is the fourth and last self-unloading lake freighter in Canada Steamship Lines (CSL) Trillium class. Like her sister ships, Baie St. Paul, Thunder Bay, and Whitefish Bay she was built in China.

Trillium-class freighter

The Trillium class is a series of freighters owned by Canada Steamship Lines (CSL). The class is divided into three subclasses; the self-discharging lake freighters, the lake bulk carriers, and the Panamax self-discharging bulk carriers. Initially a nine-ship building program, six are operated by Canada Steamship Lines for use on the Great Lakes, while three are operated by CSL Americas for international trade. Two more ships were acquired later for use by CSL Americas.