Tonnage

Tonnage is a measure of the size or cargo carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. It is incorrect to use the term "tonnage" to refer to the loaded or empty weight of the vessel itself.

Measurement of tonnage can be less than straightforward, not least because it is used to assess fees on commercial shipping.