I think there is more confusion over the definitions of weight and displacement than in any other area of Naval Architecture. Gross Registered Tonnage is actually a measurement of volume not weight and so should be refered to in a volumetric unit such as M3.

The definitions below might help to clarify the issue.

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). This calculation is complex; a hold can, for instance, be assessed for grain (accounting for all the air space in the hold) or for bales (exempting the spaces between structural frames). Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, but is still a widely used term in the industry.

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., again with differences depending on which port or country is doing the calculations). It represents the volume of the ship available for transporting freight or passengers. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

Gross Tonnage (GT) refers to the volume of all ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. It is always larger than gross register tonnage, though by how much depends on the vessel design. . It was a measurement of the enclosed spaces within a ship expressed in "tons" – a unit which was actually equivalent to 100 cubic feet.

Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.

While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:

Displacement is the actual total weight of the vessel. It is often expressed in long tons or in metric tons, and is calculated simply by multiplying the volume of the hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, first determine the volume of the submerged portion of the hull as follows: Multiply its length by its breadth and the draft, all in feet. Then multiply the product thereby obtained by the block coefficient of the hull to get the hull volume in cubic feet. Then multiply this figure by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in long tons. Using the SI or metric system : displacement (in tonnes) is volume (in m3) multiplied by the specific gravity of sea water (1.025 nominally).

The word "displacement" arises from the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship.

Lightship measures the actual weight of the ship with no fuel, passengers, cargo, water, etc. on board.

Deadweight (often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like Displacement, it is often expressed in long tons or in metric tons.