There are a myriad of terms to describe essentially how big a vessel is, and often these can be confusing. A newcomer to the world of shipping might, for instance, reasonably but erroneously believe that the Tonnage of a vessel had something to with it's weight in Tonnes.
There are however only really five terms, currently in use (GRT, NRT etc. are now obsolete), that you need to remember, and they are as follows.
Measurements of the Vessel's Size
G.T. (Gross Tonnage) - This is a measurement of the size of the ship, which calculates the area of all its enclosed spaces. It is nothing to do with the weight of the ship and is used to assess how many staff are needed to run the ship, how much port fees should be charged etc.
N.T. (Net Tonnage) - This is a measurement of the size of the cargo which can be stored on the ship, which calculates the area of all the space taken up by the cargo aboard the vessel, when the ship is fully loaded. Because the cargo will be smaller than the ship it is on or in this figure is the 'net' one.
Measurements of the Vessel's Carrying-Capacity
D.W.T. (Deadweight Tonnage) - This is the measurement of how much weight a vessel can carry. It does not include the weight of the vessel itself, but does have to take account of everything which adds weight to the vessel - cargo, passengers, crew, fuel etc.
T.E.U. (Twenty-foot Equivalent Unit) - This is the most common measurement of how much cargo a container ship can hold. A 'regular' container is twenty foot long, although many are now 'double-sized' (being 40 foot long or similar). A container ship is therfore measured in how many containers it could fit on board if they were all twenty foot long.
A Measurement of the Vessel's Weight
L.W.T. (Lightweight Tonnage) - This is how heavy the actual ship itself is. This measurement is almost never seen in shipping, because the weight on dry land of a ship is not a figure which has any importance in terms of operating the vessel, deciding how many crew it should have or how much of a port fee it should pay. However, the term is important at the beginng and end of a ship's life; when being launched and when being scrapped.
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Example Vessel Details:
THE "EMMA MAERSK"
GT : 170,974
NT : 55,396
DWT : 156,907
TEU : 14,770
LWT : Unknown