According to the International Convention on Tonnage Measurement of Ships of 1969, this is the measure of total usable capacity of ship, i.e. closed spaces (suited to transport of cargo and passengers), which were included in gross tonnage; expressed in nondimensional units with the accuracy of two decimal places.

Until year 2000, the net tonnage of ship was expressed as NRT (Net Registered Tonnage). Net Registered Tonnage - a measure of total usable tonnage of ship, i.e. closed spaces suited to transport of cargo and passengers, expressed in registered tons. 1RT /registered ton/ = 100 cubic feet = 2.83 cubic meters

• International Convention on Tonnage Measurement of Ships
  Place of publication: Dz. Urz. UE z 1983 r. Nr 56, poz. 247, z późn. zm.