What Is a Ship's Gross Tonnage?

A container ship setting out from port

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The term "gross tonnage" refers to the internal volume of a water-going vessel, and is normally used as a means for categorizing commercial vessels, especially those used for shipping. This volume measured includes all areas of the ship, from keel to funnel and from bow to stern. In modern usage, the measurement deducts the crew spaces and other parts of the ship that cannot hold cargo. Since 1969, gross tonnage has been the principal means by which a commercial ship is defined. The gross tonnage measurement has a number of legal and administrative uses. It is used to determine regulations, safety rules, registration fees, and port charges for the vessel. 

Calculating Gross Tonnage

Calculating the gross tonnage of a ship is a somewhat complicated procedure, due to the fact that most ships have an asymmetrical shape that makes calculating volume difficult. There are many ways to make this calculation, depending on the level of precision required and the agency requiring the measurement. Different formulas are used depending on the shape of the vessel, and even the types of waters on which the ship sails.

A simplified set of gross tonnage formulas is set forth by the U.S. Coast Guard Marine Safety Center, which are based on three measurements: Length (L), breadth (D), and depth (D). Under this system, the means of estimating gross tonnage is as follows: 

  • For a boat with a simple sailing hull, gross tonnage (GT) = (.5 * L * B * D) / 100
  • For a sailing boat with keel, GT = (.375 * L * B * D) / 100
  • For freighters with angled or cylindrical hulls, GT = (.67 * L * B * D) / 100
  • For ships with square barge-shaped hulls, GT = (.84 * L * B * D) / 100

The International Convention on Tonnage Measurement of Ships sets forth another, more precise formula for calculating gross tonnage of a vessel, which says GT = K * V. Here, K = .2 + .02 * log10(V), and V = interior volume of a vessel in cubic meters (m3).

History of Gross Tonnage as a Measurement Standard

Since most commercial ships were originally involved with the transport of goods, otherwise known as cartage, ships at first were rated and valued on the maximum amount of cargo that could be stuffed into every nook inside a ship. On long sailing voyages, after selling their loads of cookware, tools, machinery and other products, private traders often bought bundles of lumber, spices, cloth, and decorative goods to sell upon return to home port. Every space was stuffed full to maximize profit on both legs of the voyage, and thus each boat's value depended on just how much open space was available in the vessel.

One of the few exempt spaces in these early calculations of a ship's volume was the bilge area, where ballast was held. In early shops, no cargo could be stored here without damage since in these wooden ships the bilges were wet. Ballast stones were used on sailing ships that were leaving with a light load and returning with a heavy cargo. This might be the case when transporting a finished metal such as copper to a port where raw copper ore was loaded for the trip back to England for refining. As the lighter load was unloaded and the heavier load brought aboard, the bilge stones were removed to compensate for the extra weight. Today, piles of these foreign stones, roughly the size of bowling balls, can be found underwater near historic ports all over the world. Eventually, with the availability of mechanical pumps, water as ballast became the norm, since it was much more efficient to simply pump water in and out of the bilge to adjust the ship's weight rather than use stones or other forms of weight.  

The term tonnage originally came into usage as a means for referring to the physical space occupied by 100 cubic feet of ballast water, an amount of water that was the equivalent of about 2.8 tons. This can be confusing since a ton is usually thought of as a measurement of weight, not volume. In the context of maritime shipping, however, the term tonnage refers to the volume of space available to hold cargo.