Tonnage - International Convention on Tonnage Measurement of Ships
Tonnage measurements are governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982.
Tonnage is a measure of the cargo-carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine. In modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. Tonnage should not be confused with displacement, which refers to the actual weight of the vessel. Tonnage is commonly used to assess fees on commercial shipping.