Ask how big a cruise ship is and the answer is almost always given in tonnage. It is also almost always misunderstood. Tonnage here does not mean how much the ship weighs. It means how much enclosed space it contains — a measure of volume, worked out by formula, used to compare ships, set fees and apply safety rules. Two systems have produced that number, gross tonnage and gross register tonnage, and the change from one to the other is why the same ship can appear with two different figures in different sources.
Gross tonnage: a measure of enclosed volume
Gross tonnage, abbreviated GT, is the current international measure. It is defined by the International Convention on Tonnage Measurement of Ships, adopted by the International Maritime Organization (IMO) in 1969. The convention entered into force on 18 July 1982 for new ships, and existing ships were allowed twelve years to change over, so GT and net tonnage became the only official measures from 18 July 1994.
GT is a function of the moulded volume of all enclosed spaces of the ship, from keel to funnel. In the convention formula, GT = K1 × V, where V is that total volume in cubic metres and K1 is a coefficient that varies slightly with the volume itself. The result is dimensionless. It has no unit. It is not correctly expressed in tonnes or tons, even though older writing often adds one of those words. A figure such as 100,000 GT is an index derived from volume, chosen so the new numbers would not differ too greatly from the register figures owners and ports already knew.
That explains what GT is useful for. A larger enclosed volume generally means more cabins, public rooms and people to protect and manage. GT therefore forms the basis for manning regulations, safety rules and registration fees, and gross and net tonnage together are used to calculate port dues. It is a practical proxy for overall size, not a direct count of passenger space: two ships of similar GT can arrange that space very differently.
Open space is not counted as enclosed space, so enclosing a space in a refit can change GT even when the hull has not changed.
Gross register tonnage: the predecessor
Gross register tonnage, abbreviated GRT, was the measure GT replaced. It also described internal volume, but in register tons, where one register ton equalled 100 cubic feet (2.83 cubic metres) of enclosed space. Its lineage goes back to the system devised by George Moorsom for Britain's Board of Trade in 1854, and versions of register measurement were adopted, with variations, by different maritime states.
The crucial difference is in the words “only certain enclosed spaces.” GRT was not simply total volume divided by 100. Which spaces counted, which were exempt and which could be deducted was set out in long national rules that differed from flag to flag. Some spaces above the tonnage deck, some deckhouses and other structures could be exempt if they met particular conditions about openings and construction.
Because dues followed tonnage, a lower figure saved money on every voyage, so builders learned to design to the rules. The best-known example is the shelter deck. A light upper deck could be arranged so the space beneath it was treated as open to the weather — through tonnage openings that satisfied the letter of the exemption — and so excluded from measurement, even though in service it was useful enclosed volume. Similar ingenuity was applied to deckhouses and hatchways. It meant GRT figures were not fully comparable between countries or eras, and a universal system was needed because the register systems had drifted apart.
Net tonnage and net register tonnage
Alongside each gross figure there is a net figure, intended to describe useful or earning capacity rather than total size.
Under the old system, net register tonnage (NRT) was GRT minus deductions for spaces that did not earn revenue: propelling machinery, fuel, crew accommodation and navigation spaces. Under the convention, net tonnage (NT) is calculated independently, by a formula that is a function of the moulded volume of all cargo spaces of the ship — for a passenger ship, the spaces that carry the people the ship exists to provide for — together with factors for draught, depth and passenger numbers. The convention provides that NT shall not be taken as less than 30 per cent of GT.
Net figures appear less often in cruise writing, but they remain part of a ship's tonnage certificate and dues. An old NRT that looks small beside a later GT is usually comparing two different ideas, not reporting that the ship shrank.
Why the same ship has different figures
There are three common reasons for conflicting numbers.
First, the system changed. A ship built before 1982 was measured first in GRT and NRT, then remeasured in GT and NT during the phase-in to 1994. The two results were designed to be broadly comparable, but not identical, because GT counts all enclosed spaces by one formula and GRT counted only certain spaces under national rules. A liner quoted at 70,000 GRT in a contemporary brochure may be listed at a noticeably different GT in a modern register, with no physical change in between.
Second, the ship itself changed. Lengthening, adding a deck, enclosing a promenade or converting open space to cabins changes enclosed volume and therefore GT. A major refit can raise — occasionally lower — the figure, and responsible sources then give a new measurement from a new date.
Third, sources are often loose: brochures and newspapers have long used “tons” for both systems and repeated a launch figure long after a refit superseded it.
For those reasons this directory uses GT as its standard: it is the current measure, applied consistently to modern ships. Where the best source is a register figure, that is stated. Where a refit changed the measurement, the figure is dated, so a reader can see which version of the ship it describes.
The weight measures: displacement and deadweight
If tonnage is volume, what measures weight? Two related figures do, and both are expressed in actual tonnes.
Displacement is the weight of water displaced by the floating ship, which equals the weight of the ship and everything in it at that moment. Light displacement is the ship alone, with machinery and permanent fittings but without passengers, fuel, water and stores. Loaded displacement is that total at a permitted draught, usually the summer load line.
Deadweight tonnage (DWT) is the difference between loaded and light displacement. It is the total weight the ship can carry — passengers and cargo, plus fuel, fresh water, ballast, provisions and crew — not the weight of the ship itself. Deadweight is the headline figure for a bulk carrier or tanker, whose business is weight of cargo. For a cruise ship it is much smaller than GT, because passengers and hotel spaces are voluminous but comparatively light, and it says little about how large the ship feels.
The current benchmark in this directory, Icon of the Seas, measures 248,663 GT. That does not mean she weighs 248,663 tonnes. It means her enclosed volume, passed through the convention formula, produces that index — the largest yet recorded for a cruise ship, as the Largest cruise ships ranking shows. Her displacement is a different, smaller figure, and her deadweight smaller still. Each number answers a different question: GT, how much ship there is in volume terms; displacement, how much she weighs; deadweight, how much weight she can take aboard.
Sources: https://www.imo.org/en/about/conventions/page/international-convention-on-tonnage-measurement-of-ships.aspx https://en.wikipedia.org/wiki/Gross_tons https://en.wikipedia.org/wiki/Gross_Register_Tonnage https://en.wikipedia.org/wiki/Icon_of_the_Seas https://en.wikipedia.org/wiki/Deadweight_ton