Gross tonnage and net tonnage describe different aspects of a ship’s internal volume. Neither figure measures the vessel’s weight, and neither should be confused with deadweight tonnage or displacement.
Shipping uses several forms of “tonnage”, but they do not all measure the same thing. Gross Tonnage, Net Tonnage, Deadweight Tonnage and displacement may appear together in vessel particulars, certificates and regulations, yet each represents a different technical concept.
Gross Tonnage, or GT, describes the overall size of a ship based on the volume of its enclosed spaces. Net Tonnage, or NT, is intended to represent its useful commercial capacity, based mainly on cargo-space volume and, for passenger ships, passenger capacity.
The most important point is simple: GT and NT are not measurements of weight. A ship with a gross tonnage of 50,000 does not weigh 50,000 tonnes.
A standardised indicator of the ship’s total internal size.
A standardised indicator linked mainly to cargo spaces and passenger capacity.
What is Gross Tonnage?
Gross Tonnage is a measure of the overall size of a ship, calculated from the moulded volume of all enclosed spaces.
These spaces can include cargo holds, machinery spaces, accommodation, enclosed superstructures, stores and other enclosed areas that fall within the measurement rules.
Under the International Convention on Tonnage Measurement of Ships, 1969, GT is calculated using:
The coefficient prevents GT from being a simple one-to-one conversion of cubic metres. This is why two statements are both true:
- Gross Tonnage is based on enclosed volume.
- The GT figure is not itself expressed in cubic metres.
GT is a dimensionless index. It should be written as “50,000 GT”, not “50,000 gross tonnes”.
What is Net Tonnage?
Net Tonnage represents the useful or revenue-earning capacity of a ship. Its calculation is based mainly on the moulded volume of cargo spaces, with adjustments for the relationship between draught and moulded depth. Passenger numbers are also included when relevant.
NT therefore attempts to distinguish commercially useful capacity from the ship’s total enclosed size. Machinery spaces, crew accommodation and other non-cargo areas contribute to the ship’s overall size but do not create cargo or passenger capacity in the same way.
The complete NT formula is more complex than the GT formula because it considers:
- The volume of cargo spaces
- The ship’s moulded draught and moulded depth
- The number of passengers in cabins
- The number of other passengers the ship is certified to carry
- The ship’s calculated gross tonnage
Under the Convention, a ship’s Net Tonnage must not be calculated at less than 30% of its Gross Tonnage.
Gross Tonnage describes how large the enclosed ship is. Net Tonnage describes how much of that ship is commercially useful.
Gross Tonnage vs Net Tonnage
| Point | Gross Tonnage | Net Tonnage |
|---|---|---|
| Abbreviation | GT | NT |
| What it represents | Overall size of the ship | Useful commercial capacity |
| Main basis | Total volume of enclosed spaces | Cargo-space volume, draught relationship and passenger capacity |
| Is it weight? | No | No |
| Unit | No physical unit | No physical unit |
| Typical use | Regulatory thresholds, surveys, certification, manning and some dues | Port or canal charging systems and commercial-capacity references |
| Relative value | Normally the larger figure | Cannot be less than 30% of GT under the Convention |
A simple example
Consider a hypothetical cargo ship with the following particulars:
- Gross Tonnage: 50,000 GT
- Net Tonnage: 30,000 NT
- Deadweight: 65,000 tonnes
The 50,000 GT figure indicates the ship’s overall enclosed size under the Convention formula. The 30,000 NT figure represents its useful commercial capacity under the NT formula. The 65,000-tonne deadweight figure describes how much total weight the ship can carry before reaching its assigned load line.
The three numbers cannot be converted directly into one another because they measure different things.
These figures frequently appear together in vessel particulars and commercial negotiations. For the contractual context in which ship descriptions and capacities may be referenced, read Tide Signal’s guide to charterparties in shipping.
Remember
GT and NT are volume-based indices. DWT is a carrying-weight measurement. Displacement is the ship’s actual weight at a particular condition.
Why Gross Tonnage matters
GT is widely used as a regulatory threshold because it provides a standardised way to classify ships of very different types.
Depending on the applicable convention, national law and voyage, Gross Tonnage may affect:
- Safety and survey requirements
- Radio and GMDSS requirements
- Pollution-prevention certification
- Manning and qualification rules
- Security requirements
- Registration fees and port charges
- Whether particular IMO regulations apply
For example, several MARPOL Annex VI survey requirements apply from 400 GT, while annual operational CII requirements generally apply to relevant ships of 5,000 GT and above. Tide Signal’s guide to CII ratings and their commercial impact explains that framework in more detail. The precise rule must always be checked because ship type, voyage and other conditions may also matter.
Why Net Tonnage matters
NT is closer to the vessel’s commercial earning space, making it useful in charging and taxation systems. Port authorities, canal administrations and national tax regimes may refer to Net Tonnage when calculating dues or assessing the ship’s earning capacity.
This does not mean every port uses the same method. A port may charge on GT, NT, vessel dimensions, cargo quantity, time alongside or a combination of factors. Canal authorities may also use their own specialised measurement systems.
The relevant tariff must therefore be checked rather than assuming that one international tonnage figure determines every charge.
GT and NT are not the same as DWT
Deadweight Tonnage, or DWT, measures the maximum total weight a ship can carry between its lightweight condition and the applicable loaded displacement.
Deadweight includes more than cargo. It also includes:
- Fuel and lubricating oil
- Fresh water
- Ballast water when carried
- Stores and provisions
- Crew, passengers and their effects
DWT is expressed in metric tonnes because it is a weight measurement. GT and NT are not.
GT and NT are not displacement
Displacement is the actual weight of the ship and everything on board at a particular moment. Under Archimedes’ principle, it equals the weight of water displaced by the vessel.
A ship’s displacement changes as cargo, fuel, ballast, water and stores are loaded or consumed. Its certified Gross Tonnage and Net Tonnage do not change during normal loading and discharging.
What about GRT and NRT?
Gross Register Tonnage and Net Register Tonnage belong to older or national measurement systems. Historically, register tonnage was based on units of 100 cubic feet.
Modern GT and NT under the 1969 Tonnage Convention are calculated differently and should not be treated as direct replacements for GRT and NRT. The abbreviations may still appear in older certificates, national rules, historical vessel particulars or legacy contracts.
When comparing ships or reviewing documents, confirm which measurement system was used.
Where are GT and NT recorded?
Ships covered by the 1969 Convention are issued an International Tonnage Certificate (1969). The certificate records the ship’s Gross Tonnage and Net Tonnage and identifies the spaces included in the calculation.
The figures also appear in statutory documentation, vessel databases, port-arrival information and commercial particulars. Certificates must be valid, properly endorsed and readily available during inspections; see the Tide Signal Port State Control preparation guide.
Frequently asked questions
Can Net Tonnage be higher than Gross Tonnage?
No. NT represents a more limited measure of useful capacity derived within the ship’s overall enclosed size.
Does a higher GT mean a ship carries more cargo?
Not necessarily. GT measures enclosed size. Ship design, cargo type, deadweight, stability, hold arrangement and structural limitations determine cargo capacity.
Does GT change when the ship loads cargo?
No. Normal loading changes draught, displacement and remaining deadweight—not the ship’s certified GT.
Are containers carried on deck included in Gross Tonnage?
Deck cargo is not normally treated as an enclosed space for tonnage measurement. The ship’s fixed enclosed structure is what matters.
Which figure is used for port dues?
It depends on the port or canal tariff. Some use GT, some use NT and others apply specialised systems or cargo-based charges.
The practical difference
Gross Tonnage and Net Tonnage were created to give administrations and the maritime industry standardised measures that can be applied across different ship types.
GT measures the overall enclosed size of the ship. NT measures its useful commercial capacity.
Neither represents weight. For carrying weight, use DWT. For the ship’s actual weight, use displacement. Keeping those distinctions clear prevents one of the most common misunderstandings in vessel particulars and maritime documentation.
