Net tonnage
Net tonnage under the Tonnage Convention 1969, the earning-capacity measure derived from cargo space, and the canal net tonnages that sit alongside it.
Net tonnage is a dimensionless measure of a ship’s useful capacity, derived under the International Convention on Tonnage Measurement of Ships, 1969 from the volume of cargo spaces with corrections for draft, depth and passenger accommodation. It is intended as a proxy for earning capacity, which is why port dues and canal tolls are commonly assessed on a net rather than a gross figure.
The full article will cover the calculation and its correction factors, the floor that prevents net tonnage falling below 30 percent of gross tonnage, and the certificate. It will also treat the separate canal net tonnages, principally Suez Canal Net Tonnage and the Panama Canal Universal Measurement System, which are administered by the canal authorities under their own rules and produce different figures for the same vessel.