Register Ton

A unit of 100 cubic feet of enclosed space, entered in a ship’s certificate of registry.

The register ton is not a tonnage in its own right so much as the unit the other volumetric tonnages are counted in. Gross and net tonnage are both figures in register tons of a hundred cubic feet, and the name comes from the certificate of registry, where the number was written down.

It arrived with the Merchant Shipping Act of 1854, which adopted the system worked out by George Moorsom’s committee. The hundred cubic feet was not arbitrary: it was chosen because roughly that much hold space was reckoned to stow a ton weight of general cargo, so the new volumetric measure would produce numbers close to the weight-based tonnages shipowners were used to. Whether that equivalence ever held for any particular cargo is another matter, and it is the reason the word “ton” survived into a measure of space at all.

The ton had been a space measure from the beginning. It descends from the tun, a large wine cask, and medieval tunnage was a duty levied on the number of tuns a vessel could carry — so a ship’s size was reckoned in casks long before anyone weighed anything.

Register tons are now historical. The International Convention on Tonnage Measurement of Ships, agreed in 1969 and in force from 1982, replaced them with gross and net tonnage as dimensionless index numbers calculated from volume by formula. A modern ship’s tonnage is therefore a figure with no unit attached, which is tidier than it sounds and considerably harder to picture.