GT and NT: Tonnage, Not Weight

Anzeige

GT and NT: Tonnage, Not Weight

Gross tonnage (GT) and net tonnage (NT) are standardized measures used in ship measurement. Both are calculated according to internationally established rules and describe different aspects of a ship. GT is based on the measured volume of enclosed spaces. NT takes into account, in particular, cargo-space volume and, for passenger ships, the number of authorized passengers in specified areas. These values help classify ships according to comparable criteria and, depending on the applicable rules, provide a basis for regulations and fees.

What GT and NT indicate

GT is calculated using the formula GT = K₁ × V. Here, V is the measured volume of enclosed spaces, and K₁ is the corresponding factor. The result is a tonnage value—that is, an index based on volume. The factor K₁ itself depends on V: K₁ = 0.2 + 0.02 × log₁₀(V). For example, if V = 10,000, then K₁ = 0.28 and GT = 2,800. The convention’s formula thus converts the measured volume into a comparable figure.

NT is calculated separately using its own formula. This takes into account cargo-space volume and, for passenger ships, the number of authorized passengers in the areas covered. NT is also subject to a lower limit of 30 percent of GT. For example, if GT is 40,000, the calculated minimum is 12,000 NT. However, the actual NT value is determined by the full NT calculation; the minimum alone does not replace it. Two ships with the same GT can therefore have different NT values if their cargo-space volumes or covered passenger areas differ.

The convention identifies GT, among other things, as a basis for crewing and safety requirements, as well as registration fees. Both GT and NT may be used to calculate port fees. The value that applies in a particular case depends on the relevant rules or fee system. For comparison, it helps to consider what each measure is used for: GT measures enclosed spaces, while NT is a separate tonnage value calculated using its own formula.

How tonnage differs from mass and carrying capacity

Volume measurement and mass answer different technical questions. A GT of 30,000 does not mean a weight of 30,000 tonnes or a volume of 30,000 cubic metres. Other measures are used to determine a ship’s actual mass. These include deadweight (DWT), which is the maximum total mass of cargo, fuel, water, provisions, crew and passengers that a ship can carry up to its specified load line.

A numerical comparison makes the distinction clear: Two cruise ships can each have a GT of 30,000 yet have different DWT values, for example because their permissible loads and equipment differ. Displacement and lightship weight describe other aspects of mass and should be considered separately from tonnage. Interior layouts can also vary. GT does not directly indicate the number of cabins, restaurants or public spaces. Passenger and cabin figures are more suitable for assessing guest capacity. NT adds another independently calculated tonnage value to the comparison; which spaces are included depends on the measured areas and authorized passenger areas. For a ship’s external dimensions, see also the information on a ship’s length, breadth and depth.