Tonnage Certificate

The International Tonnage Certificate, issued under the International Convention on Tonnage Measurement of Ships 1969, states the vessel's gross tonnage (GT) and net tonnage (NT). These are dimensionless measures of the internal volume of the ship calculated according to the formulae prescribed by the Convention; they determine the port dues, canal dues, registration fees, and many regulatory thresholds — including whether SOLAS, MARPOL, and MLC apply to the vessel. For a ship's officer, tonnage determines many of the vessel's operational and regulatory parameters. SOLAS requirements for GMDSS equipment, carriage of VDR, and AIS transponder type are keyed to GT thresholds. MARPOL Annex VI applies to vessels of 400 GT and above. MLC 2006 applies to vessels of 500 GT and above on international voyages. Understanding the vessel's tonnage is therefore essential to understanding which rules apply. Gross tonnage is also the basis for the ISM Code's applicability and for the calculation of flag state fees. Net tonnage reflects the earning capacity of the vessel — the volume available for cargo and passengers — and is used for port dues and canal transit charges. The certificate is issued by the flag state after tonnage measurement and must be kept on board. Changes to the ship's configuration that affect its internal volume require remeasurement and re-issue of the certificate.