Gross Tonnage

Gross tonnage measures how big a ship is by enclosed volume rather than by weight. It is derived from the total volume of all enclosed spaces on the vessel, in cubic metres, multiplied by a coefficient defined in the International Convention on Tonnage Measurement of Ships, 1969. Because it is the product of a volume and a factor, gross tonnage is a pure number with no units — a ship is described as being, say, 50,000 GT, not 50,000 tonnes.

Gross tonnage matters because a great deal of the regulatory world is scaled to it. The size of the crew a ship must carry, the safety equipment and firefighting arrangements it needs, the certificates it must hold, and many of the thresholds in SOLAS and MARPOL are all keyed to gross tonnage. Port dues, pilotage charges and canal tolls are frequently assessed on gross or net tonnage as well, so the figure has a direct commercial impact on the cost of operating and calling at ports.

Gross tonnage is easily confused with three other measures. Net tonnage is a related volume figure that represents the ship's cargo-earning space, obtained by deducting crew, machinery and navigation spaces from the gross figure. Deadweight tonnage is a weight, not a volume, and states carrying capacity. Displacement is the actual weight of the ship. The modern GT under the 1969 Convention also replaced older 'gross register tonnage' figures, which used a different formula, so historical comparisons need care. In everyday use, gross tonnage answers the question 'how large is this ship?' in a way that governs its paperwork, its manning and its bills, while deadweight answers 'how much can it carry?'.