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register tonnage[′rej·ə·stər ′tən·ij]
of a vessel, the volume of the internal areas of a merchant vessel. Register tonnage serves as a description of the size of a vessel and is figured in register tons (1 register ton equals 100 cubic feet or 2.83 cu m).
A distinction is made between gross register tonnage, which is the volume of all internal ship areas with the exception of a few ballast tanks and so forth, and net register tonnage, which is the volume of the space available for cargo and passengers. Register tonnage is confirmed by a certificate of measurement issued by a classification society. The rules for determining register tonnage are established by international or local rules for the measurement of vessels, such as the rules of the Panama or Suez canals. The taxes and duties charged for ships, including those for use of moorings or for pilot service, depend on the register tonnage.