Net Tonnage

Net tonnage is a measure of the earning capacity of a ship expressed as a volume, and like gross tonnage it is a dimensionless number rather than a weight. It is calculated under the International Convention on Tonnage Measurement of Ships from the ship's molded volume of cargo spaces, taking the gross tonnage and deducting the volumes that cannot be used to carry cargo — the spaces occupied by machinery, crew accommodation, navigation and other non-cargo areas. What remains represents the useful, revenue-earning space of the ship, which is why net tonnage is regarded as a measure of a vessel's commercial capacity.

Because it reflects earning capacity, net tonnage is widely used as the basis for charges levied on ships. Port dues, harbour and light dues, pilotage and towage fees, and the tolls charged by canals such as Panama and Suez are frequently assessed on net tonnage, or on a combination of gross and net tonnage, on the reasoning that a ship able to earn more from a port call can fairly be charged more for using its facilities. This gives net tonnage a direct and continual commercial significance in the operation of a ship.

Net tonnage should be distinguished from the other measures with which it is easily confused. Gross tonnage measures the total enclosed volume of the ship and is used for regulatory thresholds, manning and safety requirements. Deadweight tonnage is a weight, stating how much cargo, fuel and stores the ship can lift. Displacement is the actual weight of the ship. Net tonnage, being a volume of cargo space, sits alongside gross tonnage as one of the two volumetric tonnage figures on a ship's certificates, and the older term net register tonnage refers to the figure calculated under earlier rules. For anyone dealing with the costs of operating and calling at ports, net tonnage is a figure of everyday importance.