Don’t confuse Gross Tonnage with the weight of the vessel. Tonnage, Net and Gross, is a volume calculation.
Net Tonnage: is a vessel's gross tonnage minus deductions of space occupied by accommodations for crew, by machinery, for navigation, by the engine room and fuel. A vessel's net tonnage expresses the space available for the accommodation of passengers and the stowage of cargo. A ton of cargo in most instances occupies less than 100 cubic feet; hence the vessel's cargo tonnage may exceed its net tonnage, and, indeed, the tonnage of cargo carried is usually greater than the gross tonnage.