Gross Register Tonnage (GRT) represents the total internal volume of a vessel, with some exemptions for non-productive spaces such as crew quarters; 1 gross register ton is equal to a volume of 100 cubic feet (2.83 m³), which volume, if filled with water, would weigh around 2,800 kg or 2.8 tonnes.

Net Register Tonnage (NRT) is the volume of cargo the vessel can carry; ie. the Gross Register Tonnage less the volume of spaces that will not hold cargo (e.g. engine compartment, helm station, crew spaces, etc)

Deadweight tonnage (DWT) is a measure of how much mass or weight of cargo or burden a ship can safely carry. Deadweight tonnage at any given time is defined as the sum of the weights or masses of cargo, fuel, fresh water, ballast water, provisions, passengers and crew.