This calculation of gross register tonnage is complex; a hold can, for instance, be assessed for grain (accounting for all the air space in the hold) or for bales (exempting the dead space between the ship's ribs). Also, certain non-productive spaces, such as crew's quarters, are exempted from the calculation.
Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, but is still a widely used term in the industry.
Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, Gross Tonnage should be used in day to day business instead of Gross Register Tonnage. Gross Register Tonnage is a function of the all the moulded volume of all enclosed spaces of the ship.