And most importantly why these are not a measure of how much the ship weighs.
When discussing ships, people often like to compare ships to one-another. Frequently we can use which ship is longer; or which ship has more cabins; or which ship carries more passengers and crew. A lot of people will also use the term “ton” as well since this is the technical industry term for gauging the true size of a ship. Frequently people also misuse this term so this post will clarify once and for all what these terms are, what they mean, and why they’re used.
In short, despite the use of the word “ton,” these measurements do not refer to the actual weight of the vessel. Instead, tonnage refers to various dimensions of a ship’s internal volume or carrying capacity, used for regulatory, operational, and commercial purposes. The main ones we will cover in this post are Gross Tonnage (GT), Gross Registered Tons (GRT), Net Tonnage (NT), and Deadweight Tons (DWT). As a result, you will be an expert in these next time you’re on a ship and also you’ll be able to see why these do not represent the physical weight of a ship.
Gross Tonnage (GT)
Gross Tonnage (GT) is the most widely used measure for determining the overall size of a ship. It is a dimensionless figure, calculated based on the internal volume of the vessel, and not its weight.
- Calculation: Gross Tonnage is calculated by multiplying the total volume of the ship’s enclosed spaces (in cubic meters) by a constant multiplier. This measurement is governed by the International Convention on Tonnage Measurement of Ships (1969), which simplified earlier tonnage calculations. The current calculation is simply
GT=K×V
Where:
- K is a constant derived from a formula depending on the ship’s volume.
- V is the total volume of all enclosed spaces.
Gross Tons is primarily used for regulatory purposes, including classification and port dues primarily for passenger ships and container ships. The higher the GT, the higher the fees. Gross Tonnage does not represent cargo capacity, but is a measure of the overall size of the entire ship in relation to its enclosed volume. There’s also further details in the guidelines about what determines enclosed space. Areas that are surrounded by the ships superstructure but open at the top do not count such as the Central Park of Royal Caribbean’s Oasis and Icon-class vessels.
Gross Registered Tons (GRT)
Gross Registered Tons (GRT) was an older unit used before the advent of the Gross Tonnage system above. It also refers to the internal volume of a ship, but with a slightly different methodology for determining the measurement.
- Calculation: GRT is based on the total volume of all enclosed spaces within the ship, but in this case, one GRT equals 100 cubic feet of space.
1 GRT=100 cubic feet of internal
GRT was primarily used for administrative purposes like registration, safety regulations, and port dues. It was eventually replaced by the Gross Tonnage measurement under the International Tonnage Convention because it was less precise and somewhat inconsistent.
Net Tonnage (NT)
Net Tonnage (NT) is another volumetric measure, which focuses more on a ship’s earning potential. Unlike Gross Tonnage, which accounts for all enclosed spaces, Net Tonnage only considers spaces that can be used to carry cargo, or other revenue-generating activities.
- Calculation: NT is derived from the Gross Tonnage but subtracts non-revenue spaces like the engine room, crew quarters, and other spaces necessary for the operation of the ship.
NT = K2Vc(4d/3D)2 + K3(N1+N2/10)
Where a lot of these variables are related to items on the ship such as the draft, depth, and shape of the hull.
NT is often used to determine port fees, canal transit dues (such as through the Panama or Suez Canals), and other shipping costs related to commercial operations.
Deadweight Tonnage (DWT)
Deadweight Tonnage (DWT) is distinct from the previous terms in that it relates to the weight a ship can carry, but it still does not indicate the weight of the ship itself. DWT measures the total weight that a ship can safely carry, including cargo, fuel, passengers, crew, provisions, and ballast water.
- Calculation: DWT is the difference between the displacement of a fully loaded ship (the ship’s total weight when submerged to its maximum draft) and the displacement of the ship when empty (light displacement).
DWT = Displacement (fully loaded)− Light displacement
DWT is an essential figure for commercial shipping as it defines the maximum amount of cargo and other operational loads the vessel can carry safely. This is most commonly used for oil and product tankers. It is a crucial figure for compliance with safety regulations and for calculating port expences.
Key Takeaway: Not a Measure of Weight
It is critical to understand that none of these tonnage terms refer to the actual weight of the ship:
- Gross Tonnage (GT): Measures total internal volume.
- Gross Registered Tons (GRT): Old system, also measures volume.
- Net Tonnage (NT): Measures cargo carrying internal volume.
- Deadweight Tonnage (DWT): Refers to the weight the ship can carry.
The physical weight of a ship is measured by its displacement, which is the weight of the water the ship displaces when floating.
Understanding these distinctions is crucial for professionals in maritime industries, as tonnage affects everything from fees and regulations to operational logistics and safety protocols.