Insights

Tonnage Measurement: GT, NT and the ITC 1969

Much of maritime regulation, fees and manning scales with a ship's tonnage, so how tonnage is measured matters. The International Convention on Tonnage Measurement of Ships, 1969 (in force from 1982) created a single, universal system, replacing the patchwork that came before.

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Gross and net tonnage

Despite the name, tonnage is a measure of volume, not weight. Gross Tonnage (GT) reflects the total enclosed volume of the ship and is used widely as a size threshold in conventions such as SOLAS and MLC. Net Tonnage (NT) reflects the volume available for cargo and passengers and often drives port dues and fees.

The certificate behind the numbers

A ship's GT and NT are calculated under the Convention's formulas and recorded on its International Tonnage Certificate (1969). Because so many requirements switch on at specific GT thresholds, an accurate tonnage figure is foundational to the rest of a vessel's compliance.

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