Gross tonnage vs deadweight

Open the spec sheet of any merchant ship and you will meet a confusing pair of figures. One is the gross tonnage, the other the deadweight tonnage, and both are quoted in something that sounds like tonnes. People reach for them interchangeably all the time — and almost always wrongly. They describe entirely different properties of the ship, and only one of them is a weight at all.

Gross tonnage: a measure of volume

Gross tonnage is, in plain terms, a measure of how much enclosed space a ship has. It is defined under the 1969 International Convention on Tonnage Measurement of Ships, which standardised the figure worldwide so that one ship's GT means the same thing as another's. The formula is deceptively simple:

GT = K1 × V — where V is the total enclosed volume of the ship in cubic metres, and K1 is a multiplier of roughly 0.2 to 0.28 that rises gently with the size of the ship. 1969 International Convention on Tonnage Measurement of Ships, Annex I, Regulation 3

The critical point: gross tonnage has no units. It is not tonnes, not cubic metres, just a dimensionless index derived from volume. A "60,000 GT" ship is not 60,000 tonnes of anything — it is a ship of a certain enclosed size. Because GT scales with the volume a vessel encloses rather than what it can lift, it is the figure regulators reach for: it drives manning requirements, safety convention thresholds, port dues and canal fees, all of which broadly track the size of the ship.

Net tonnage

A close relative is the net tonnage (NT), also defined under the 1969 Convention. NT is derived from the volume of the cargo and passenger spaces only — the earning volume — after deducting machinery, crew and other non-revenue compartments. It is the figure most directly tied to a ship's commercial usefulness, and it too is used to set many dues. GT is the whole enclosed ship; NT is the part that earns money.

Deadweight tonnage: a measure of weight

Deadweight tonnage answers a completely different question: how much weight can this ship carry? DWT is a true weight, expressed in metric tonnes, and it covers everything the ship loads on top of itself — cargo, fuel, fresh water, stores, ballast, provisions and crew. Formally, it is the difference between two displacements:

• Loaded displacement — the weight of water the ship displaces when loaded to its summer load line.
• Lightship — the weight of the ship itself, empty: hull, machinery and permanent equipment, with no cargo, fuel or stores.

Deadweight is loaded displacement minus lightship. It is the single best measure of a cargo ship's carrying capacity, which is why a tanker or bulk carrier is almost always described by its DWT — a "300,000 DWT VLCC" can lift about 300,000 tonnes of cargo and consumables.

Why they are not interchangeable

Because GT tracks volume and DWT tracks weight, the two diverge wildly depending on what a ship is built to do. A vessel that encloses a great deal of light, airy space but carries little weight has a high GT and a low DWT. A vessel that is mostly a hull full of dense liquid has the opposite profile. The clearest contrast is a cruise ship against a very large crude carrier:

Ship type	Gross tonnage (GT)	Deadweight (DWT)
Large cruise ship	~230,000	~20,000 t
VLCC crude tanker	~160,000	~300,000 t

The cruise ship encloses vast volume — thousands of cabins, atria, theatres and decks — so its GT is enormous, yet it carries comparatively little weight, so its DWT is modest. The tanker is the mirror image: its enclosed volume is smaller relative to its size, but it lifts a colossal weight of oil, so DWT towers over GT. Quote one figure where the other is meant and the picture of the ship collapses.

Displacement and lightship in one line

To keep the family of terms straight: displacement is the actual weight of the ship at any moment (equal to the weight of water it pushes aside); lightship is its displacement empty; and deadweight is the gap between the two — the weight it is free to load. GT and NT sit apart from all of these, measuring volume rather than weight. You will find every one of these figures listed on a vessel's record, defined in plain language in the Marifest glossary.

Which figure to use, and where

Different ship types are conventionally sized by different figures, and getting this right is half of reading a fleet correctly. Tankers and dry bulk carriers are quoted in DWT; container ships in TEU; passenger and cruise ships in GT and berths. We walk through these conventions for every major category in ship types explained. GT and DWT also feed directly into emissions ratings — the CII rating uses a ship's carrying capacity (DWT for cargo ships, GT for passenger and cruise vessels) as the denominator when measuring carbon intensity per tonne-mile.

Whichever figure you need, you can read both straight from the record. On Marifest you can search any of 97,000+ vessels by name or IMO number in the open registry and see GT, NT, DWT, lightship and dimensions side by side — so you never have to guess which "tonnage" a source meant.