Gross tonnage vs deadweight — what ship sizes actually measure

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.