Gross Tonnage is a measure of volume inside a vessel. This includes all areas from keel to funnel and bow to stern.
The means to calculate gross tonnage is laid out in the International Convention on Tonnage Measurement of Ships.
Measurement of the total interior volume of a vessel in cubic meters is multiplied in a formula by a second factor which is determined by the volume of the vessel.
The formula is:
Gross Tonnage (GT) = K*V
Where V = Interior volume of a vessel in cubic meters
Gross Tonnage is used to determine the number of crew, safety rules, registration fees, and port dues. It is the standard most often used to define a vessel.