Gross tonnage (GT)

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Gross tonnage (GT) is a measure of a ship’s overall internal volume, calculated under the IMO’s 1969 Tonnage Convention from the total enclosed spaces of the vessel. Despite the name, it is not a weight and has no unit — it is a dimensionless index derived from volume in cubic metres.

Gross tonnage matters because so much regulation keys off it: safety requirements, manning levels, registration and port fees, and the application thresholds of major conventions (for example, SOLAS applies broadly to ships of 500 GT and above, and the ISM Code to cargo ships of 500 GT and above). Net tonnage, derived from the volume of cargo spaces, is used mainly for harbour and canal dues.

For describing commercial carrying capacity, the industry instead uses deadweight tonnage for bulk ships and tankers, and TEU for container vessels. Passenger ships, however, are conventionally described by gross tonnage — which is why cruise ship “size” figures are GT numbers.