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Glossary Term

Gross Tonnage (GT)

Gross tonnage (GT) is a measure of a ship's overall size, calculated from the total volume of her enclosed spaces. Despite the name, it is not a weight. It is determined under the International Convention on Tonnage Measurement of Ships, 1969 (ITC 1969), and many yacht rules use it as a size threshold.

How gross tonnage is calculated

Regulation 3 of Annex I to the convention gives the formula GT = K1 × V. V is the total volume of all enclosed spaces in cubic metres, and K1 = 0.2 + 0.02 log10 V; the convention also tabulates K1 in an appendix. Because K1 increases with volume, GT is not a fixed fraction of the enclosed volume.

Enclosed spaces are all spaces bounded by the hull, by fixed or portable partitions or bulkheads, and by decks or coverings other than permanent or movable awnings. An opening in a deck or bulkhead, or a missing bulkhead, does not by itself take a space out of the enclosed volume.

Regulation 2(5) lists excluded spaces that are open to the weather in defined ways. For example, a space in a deck erection immediately below an uncovered opening in the deck above can be excluded if the opening is exposed to the weather, but only within the area of that opening. An excluded space is counted as enclosed if it is fitted with shelves or other means for securing cargo or stores, if its openings are fitted with any means of closure, or if its construction allows those openings to be closed. On a yacht, whether a covered deck area counts towards GT can therefore depend on how it is built and fitted.

Gross tonnage, net tonnage and displacement

The convention defines net tonnage (NT) as the measure of a ship’s useful capacity. It is calculated with a separate formula that takes account of cargo spaces and, where 13 or more are permitted, passengers, and it may not be taken as less than 30 per cent of GT. Displacement is different again: it is the weight of the yacht, and the tonnage convention does not measure it.

Some rules still refer to older measurements. For yachts built before 21 July 1968, the application clauses of the previous LY3 code and REG Yacht Code Part A include an alternative size criterion of 150 gross tons or over, alongside the 24-metre load line length criterion. Those gross tons use the tonnage rules in force on 20 July 1968, a different measurement from GT under ITC 1969.

Who determines it and which yachts are covered

The convention applies to ships on international voyages registered in a contracting State or a territory to which it applies, and to unregistered ships flying a contracting State’s flag. It excludes warships and ships under 24 metres in its own waterline-based length, rather than length overall. It also excludes ships navigating solely on the Caspian Sea, the Great Lakes of North America and the St Lawrence River, or the Plate, Parana and Uruguay rivers, within the limits the convention specifies. For a yacht outside the convention, confirm the applicable tonnage measurement rules with the flag Administration.

The flag Administration determines gross and net tonnage and may entrust the work to persons or organisations it recognises, but it remains fully responsible for the result. A ship whose tonnages have been determined under the convention is issued with an International Tonnage Certificate (1969).

Why the figure matters on a yacht

Several regimes use GT as a dividing line:

  • The MLC defines gross tonnage by reference to Annex I of ITC 1969, with an exception for ships covered by the IMO tonnage measurement interim scheme: it uses the tonnage in the certificate’s Remarks column. For covered ships flying a ratifying State’s flag, a maritime labour certificate is required at 500 GT or over when making international voyages or operating from a port, or between ports, in another country.
  • For covered ships flying a ratifying State’s flag that are under 200 GT and not making international voyages, the MLC allows the competent authority, after consultation with the shipowners’ and seafarers’ organisations concerned, to disapply certain Code details if applying them is not currently reasonable or practicable and the subject is dealt with differently by national laws, regulations, collective bargaining agreements or other measures.
  • The previous LY3 code and its successor, REG Yacht Code Part A, set separate requirements for yachts within their scope below 500 GT and those of 500 GT and over in several technical chapters, including machinery, steering gear, bilge pumping and fire protection. The applicable edition and transitional provisions should be checked with the flag Administration.

Two yachts of the same length can therefore fall under different requirements if their enclosed volumes differ.

Alterations, refit and change of flag

Subject to the exceptions in its regulations, Article 10 of the convention provides that an International Tonnage Certificate ceases to be valid, and is cancelled by the Administration, if alterations to the ship’s arrangement, construction, capacity, use of spaces, permitted number of passengers, assigned load line or permitted draft necessitate an increase in gross or net tonnage. Enclosing an open deck area, adding a deckhouse or extending the superstructure can have that effect if it adds enclosed volume.

A tonnage increase can also move a yacht into a different regulatory band. Under REG Yacht Code Part A, where a change in a yacht’s particulars reaches a new regulatory threshold such as 500 GT, the relevant provisions for a new vessel of that size apply, and detailed proposals are to be submitted to the Administration at the earliest opportunity. The effect of a planned alteration on GT is best confirmed with the flag Administration or its recognised organisation before the design is fixed.

A certificate also ceases to be valid when the ship transfers to another flag. If the new flag State is a contracting State, the existing certificate stays in force for up to three months, or until the new Administration issues a replacement, whichever is earlier.

Frequently asked questions

Can a yacht’s GT be reduced after she is built?

GT follows the volume of enclosed spaces, so it changes only if that volume changes, for example when an enclosed space is removed or rebuilt so that it meets the conditions for an excluded space. The flag Administration then determines the tonnage again for a new certificate. The twelve-month waiting period in Regulation 5 of the convention applies to a decrease in net tonnage, not gross tonnage.

A lower GT does not by itself remove requirements a yacht already meets. Under REG Yacht Code Part A, a yacht undergoing repairs, alterations or modifications must continue to comply, as a minimum, with the requirements previously applicable to her.

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