What is Deadweight Tonnage (DWT)?
Deadweight Tonnage (DWT) represents the total weight of the maximum cargo a ship can carry, along with fuel, fresh water, crew, and equipment. DWT is an important indicator used to measure a ship’s capacity and transport capability. While there are other units used to understand the size and capacity of a ship, such as Gross Tonnage (GT) and Net Tonnage (NT), DWT is typically the most commonly used unit to measure a ship’s cargo carrying capacity.
How is Deadweight Tonnage Calculated?
Deadweight Tonnage is calculated as the weight difference between a ship when it’s fully loaded and when it’s empty. This is expressed in the formula:
DWT = Fully Loaded Displacement – Empty Displacement
Where:
Fully Loaded Displacement: The weight of the volume of water displaced by the ship when it’s fully loaded. Empty Displacement: The weight of the volume of water displaced by the ship when it’s empty.
What are GT and NT on a Ship?
GT (Gross Tonnage):
Gross Tonnage represents the total internal volume of a ship. However, this value is not directly related to the weight of the cargo a ship can carry; it rather represents the total empty space or volume inside the ship. GT is the sum of all enclosed spaces on a ship and is often used as an indicator of the ship’s size. The calculation of GT follows international rules, and these rules are determined by the 1969 International Tonnage Measurement Convention (ITC69).
NT (Net Tonnage):
Net Tonnage represents the total empty space available on a ship for carrying cargo. In other words, NT is a measure of the usable internal volume that signifies a ship’s cargo-carrying capacity. However, it shouldn’t be confused with deadweight tonnage (DWT); while DWT represents the maximum weight of cargo a ship can carry, NT signifies the usable internal volume. The calculation of NT also follows the guidelines set in ITC69 and is typically determined by subtracting certain enclosed spaces (e.g., the engine room) from the gross tonnage (GT).
GT and NT values are essential during the design, construction, and trade of a ship and are typically used as a basis for port fees, canal fees, and other maritime costs. These values are just two of many factors affecting a ship’s commercial operations.