The International Convention on Tonnage measurements of ships (ITC69) is an IMO Convention that is applicable for all vessels engaged in international trade, with the following exceptions: ships of war; ships of less than 24 meters in length.

The tonnage of a ship shall consist of gross tonnage (GT) and net tonnage (NT), which are given on the International Tonnage Certificate (1969).
Gross tonnage (GT) is the measure of a vessel’s enclosed spaces. GT forms the basis for manning regulations, safety rules, port dues and registration fees.

Net Tonnage (NT) is a measure of the volume of a vessel’s cargo holds, used to calculate port dues. Both of these are measures of volume and not to be confused with Deadweight (DWT) – which is the largest weight of cargo, bunkers and stores a ship is able to carry.