tonnage

Tonnage is a measure used to express the size and carrying capacity of ships, or more generally the capacity of vessels and other equipment. It is not a direct measure of weight; rather, it reflects volume-based and weight-based capacity concepts that affect regulation, fees, and operations.

The modern merchant-ship system uses Gross Tonnage (GT) and Net Tonnage (NT). GT is a function of the ship’s total enclosed volume and is a dimensionless figure used for regulatory purposes such as safety rules and manning requirements. NT reflects the cargo-carrying portion of that volume, again a dimensionless figure used for responsibilities like port dues and efficiency assessments. Together, GT and NT provide standardized indicators of a ship’s size and potential cargo handling capacity.

Historically, ships were measured using Gross Register Tonnage (GRT) and Net Register Tonnage (NRT), based on the notion that one ton represented 100 cubic feet of space. The 1969 International Convention on Tonnage Measurement modernized the system, replacing GRT and NRT with GT and NT to offer a more consistent and scalable approach to ship size.

Deadweight Tonnage (DWT) is a separate, weight-based measure of a vessel’s carrying capacity. It represents how much weight a ship can safely carry, including cargo, fuel, crew, and provisions, and is calculated as the difference between fully loaded displacement and the ship’s lightweight displacement, expressed in metric tons.

In general usage, tonnage can also appear as a loose reference to weight in tons (metric tonnes, short tons, or long tons). It is important to distinguish tonnage as a capacity or regulatory metric from the actual weight of the ship or its cargo.