Tonnage is the amount of weight a commercial vessel can carry, usually as determined by an official authority for purposes of taxes or duties imposed for entering certain ports.
There are five categories by which tonnage of a vessel is calculated: registered, gross register, net register, displacement and deadweight tonnage.
Registered Tonnage is stated in tons of 100 cubic feet per ton. This is the total internal capacity of a vessel.
Gross Register Tonnage is the same as Registered Tonnage.
Net Register Tonnage is the part of the Registered Tonnage which is used to earn money. All spaces which are not revenue producing are deducted from the registered tonnage.
Displacement Tonnage is the actual weight of the ship and is equal to the weight of water displaced by the vessel. Measured in long tons (2,240 pounds per ton).
Deadweight Tonnage is the carrying capacity of a vessel measured in long tons.
