Ship size is measured by volume as well as by weight. Gross tonnage is a dimensionless index of a ship’s overall internal volume, calculated according to the International Convention on Tonnage Measurement of Ships.

It is used for manning regulations, safety rules, registration fees, and some canal tolls. Gross tonnage is not a weight; it is derived from moulded volumes of the hull and enclosed spaces with a mathematical formula. Net tonnage, by contrast, reflects volume available for cargo and passengers.

Because many regulations trigger at certain gross tonnage thresholds, designers sometimes optimize internal volume to stay just below a limit. Officers and agents use the certified gross tonnage when dealing with authorities and service providers worldwide.

• Index of a ship’s total internal volume
• Calculated by international formula, not a weight
• Used for regulations, fees, and tolls
• Distinct from net tonnage and deadweight