Gross Tonnage (GT)
Gross Tonnage (GT) is a measure of a ship's total enclosed internal volume, calculated by a formula in the International Tonnage Convention. Despite the name it is a volume measure, not a weight — a dimensionless number derived from the ship's cubic capacity.
GT is the figure that drives much of a ship's regulatory and commercial life: safe-manning requirements, which conventions apply, registration and class fees, and many port dues, canal tolls and pilotage charges are all scaled to gross tonnage. Its companion, Net Tonnage (NT), measures the volume available for cargo and passengers. GT appears on the tonnage certificate and is one of a ship's core identifying particulars.
Gross tonnage quietly sets a large share of a ship's running costs — port dues, canal tolls, pilotage, manning and fees all key off GT. It also decides which regulations bite. Getting GT-based charges right across a fleet is a real cost-control exercise, which is why the figure sits at the heart of vessel administration.
Is gross tonnage a weight?
No — it is a measure of the ship's total enclosed volume, expressed as a dimensionless number.
What is gross tonnage used for?
Manning rules, applicable regulations, registration and class fees, and port dues, canal tolls and pilotage charges.