Gross register tonnage is a measure of the internal volume of a ship expressed in units of 'register tons'. Gross register tonnage (GRT, grt, g.r.t.) represents the total internal volume of a vessel, while Net register tonnage accounts for certain nonproductive spaces and is always lesser in value. A register ton is equal to a volume of 100 cubic feet (~2.83 m³).^{[1]}^{[2]}
Gross register tonnage is not a measure of the ship's weight or displacement and should not be confused with terms such as gross tonnage, deadweight tonnage, net tonnage, or displacement.^{[1]}^{[3]}
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).
It has been superseded by Gross Tonnage and is not used or even calculated for modern ships.

