Gross tonnage vs deadweight: how ship size is measured
· 3 min read
In short
Gross tonnage (GT) measures how big a ship is inside: it is calculated from the volume of all its enclosed spaces and has no unit. Deadweight (DWT) measures how much a ship can carry: the weight in tonnes of cargo, fuel, water, stores and crew that brings it down to its summer load line. A cruise ship is huge inside and carries little weight, so its GT is high and its DWT low; a tanker is the opposite.
Gross tonnage: a measure of volume
Gross tonnage comes from the International Convention on Tonnage Measurement of Ships, 1969, which entered into force on 18 July 1982. It is calculated from V, the total volume of all enclosed spaces in cubic metres:
GT = K1 × V, where K1 = 0.2 + 0.02 × log10(V)
The logarithm makes the figure grow a little faster than the volume, and the result is a dimensionless number, not a weight and not cubic metres. A ship of 100,000 cubic metres enclosed volume has a gross tonnage of 30,000. The old “gross register tons”, which were units of 100 cubic feet, were replaced by this system.
Gross tonnage matters because rules are written against it. SOLAS, for example, requires AIS on ships of 300 GT and above on international voyages, and the IMO number scheme applies from 100 GT. Registration fees, manning rules and many port dues are also set by GT. Net tonnage (NT) is the companion figure for the volume of the cargo spaces only; the Convention does not let it fall below 30% of the gross tonnage.
Deadweight: a measure of weight
MARPOL defines deadweight as “the difference in tonnes between the displacement of a ship in water of a relative density of 1.025 at the load waterline corresponding to the assigned summer freeboard and the lightweight of the ship”. In plain terms:
- Displacement is the total weight of the ship and everything in it, equal to the weight of the water it pushes aside.
- Lightweight is the weight of the empty ship: hull, machinery and equipment, with no cargo, fuel, water or stores.
- Deadweight is the difference at the summer load line in seawater: everything the ship can load, in metric tonnes.
Deadweight is what charterers and cargo owners care about, and it defines size classes such as VLCC or Capesize. For container ships the capacity is usually quoted in TEU (twenty-foot equivalent units) as well.
Why the two numbers diverge
A cruise ship is almost all enclosed volume (cabins, theatres, atriums) and carries little weight, so its GT is very large and its DWT modest. A tanker or bulk carrier is mostly a heavy cargo hold, so its DWT can exceed its GT. Container ships sit in between. The live table shows real ships from Voydar’s data.
Gross tonnage and deadweight side by side
Live from VoydarThe three largest passenger ships, cargo ships and tankers by gross tonnage that Voydar heard on AIS in the last 30 days and holds both figures for.
| Ship | AIS type | GT | DWT (t) | DWT ÷ GT |
|---|---|---|---|---|
| LEGEND OF THE SEAS | Passenger | 250,800 | 21,513 | 0.09 |
| STAR OF THE SEAS | Passenger | 248,663 | 20,964 | 0.08 |
| ICON OF THE SEAS | Passenger | 248,663 | 21,513 | 0.09 |
| CMA CGM MONTMARTRE | Cargo (hazardous cat. A) | 237,200 | 216,900 | 0.91 |
| CMA CGM LOUVRE | Cargo (hazardous cat. A) | 237,200 | 216,900 | 0.91 |
| CMA CGM RIVOLI | Cargo (hazardous cat. A) | 237,200 | 216,900 | 0.91 |
| SA EUROPE | Tanker | 234,006 | 441,561 | 1.89 |
| SA OCEANIA | Tanker | 234,006 | 441,585 | 1.89 |
| OLYMPIC LION | Tanker | 170,611 | 319,541 | 1.87 |
GT and DWT as held in the vessel record (sources shown on each ship’s page). The DWT ÷ GT ratio is computed by Voydar.
Which one to use
- Comparing how big ships are, especially passenger ships: gross tonnage.
- Comparing what cargo ships can carry: deadweight (or TEU for container ships).
- Checking which rules apply: gross tonnage, since IMO conventions set their thresholds in GT.
- Comparing fleets and registries: both are used; UNCTAD’s statistics on flags are mostly by deadweight.
Every Voydar vessel page shows GT and DWT when a source provides them, with the source next to each figure. Browse by ship type to compare ships of the same kind.
Questions and answers
- Is gross tonnage a weight?
- No. Despite the name, gross tonnage is calculated from volume and has no unit. Deadweight and displacement are weights, in tonnes.
- What does DWT stand for?
- Deadweight tonnage, the weight in tonnes of everything a ship can carry: cargo, fuel, fresh water, stores and crew.
- Why do cruise ships have such a high gross tonnage?
- Because gross tonnage measures enclosed volume, and a cruise ship encloses a great deal of space for passengers. Its deadweight, the weight it carries, is small by comparison.
Look up a ship
Search any vessel by name, IMO number or MMSI for its free Voydar profile: live position, identity history, ownership, inspections and sanctions.
Sources
Primary sources for the facts in this guide. Figures labelled as Voydar data come from Voydar’s own records, with their method stated next to them.
- IMO: International Convention on Tonnage Measurement of Ships, 1969
- IMO: MARPOL Annex I (definition of deadweight, regulation 1)
- UNCTAD: Review of Maritime Transport
Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.