Gross tonnage vs deadweight: what ship size measures

Gross tonnage is not a weight. It is a unitless index of a ship's total enclosed volume, calculated from a formula in the 1969 tonnage convention, which is why it suits cruise ships, whose product is interior space. When Icon of the Seas is called 248,663 GT, nothing about that number involves scales.

Deadweight (DWT) is a mass: the tonnes of cargo, fuel, stores and crew a ship can lift before reaching her load line. It is the honest measure for tankers and bulk carriers, whose product is carrying capacity, Seawise Giant's 564,763 DWT means she lifted over half a million tonnes. Displacement, a third measure, is the actual weight of ship plus everything aboard, the water she pushes aside, and is mostly quoted for warships and curiosities like Prelude's 600,000 tonnes fully ballasted.

Older ships complicate things further: gross register tonnage (GRT), used before 1982, measured volume in 100-cubic-foot units under different rules, so Titanic's 46,328 GRT is not directly comparable to a modern GT figure. This site never ranks across units, cruise ships rank by GT, container ships by TEU, tankers and bulkers by DWT, and flags GRT wherever it appears.