What is deadweight tonnage?
Deadweight tonnage (DWT) is the total weight a vessel can carry at its summer load line: cargo plus bunkers, fresh water, stores, crew and constants. It equals the ship’s loaded displacement minus her lightship weight, and it is expressed in metric tonnes.
DWT is the single most important number on a dry bulk carrier’s particulars, because it caps how much the ship can lift on any given voyage. It is a measure of carrying capacity by weight, not a measure of the cargo alone and not a measure of internal volume. The classification societies that survey and certify the fleet, Lloyd’s Register, DNV and ABS, record deadweight alongside displacement and lightship in the ship’s stability and tonnage documentation, and Clarksons Research carries deadweight as the headline field when the desk sizes a stem.
Because deadweight is fixed at the summer load line under the International Convention on Load Lines (the 1966 Load Lines Convention, as amended), it is a regulatory ceiling rather than a commercial estimate. A vessel cannot lawfully load past the marks that correspond to her summer deadweight in salt water, which is why the figure anchors both the safety case and the freight calculation. The rest of this page works through how the number is derived, how it differs from the adjacent measures it is routinely confused with, and how it shifts with water density and seasonal marks.
How deadweight tonnage is calculated
Deadweight is a difference between two displacements: the weight of water the hull pushes aside when fully loaded to her marks, and the weight of water she pushes aside light. The properties and the governing formulas are set out below.
| Property | Value / Formula | Unit | Reference |
|---|---|---|---|
| Deadweight (DWT) | loaded displacement minus lightship | tonnes | Lloyd's Register / DNV / ABS stability documentation |
| Loaded displacement | underwater volume at summer mark x water density | tonnes | Archimedes' principle |
| Lightship | weight of the empty vessel: hull, machinery, permanent fittings | tonnes | Builder's lightship survey, certified by class |
| Summer deadweight | DWT at the summer load line in salt water | tonnes | International Convention on Load Lines 1966 |
| Design deadweight | DWT at the design (scantling or contract) draught | tonnes | Builder's specification, class-approved |
| Underwater volume | moulded volume of hull below the waterline | cubic metres | Hull lines plan, class-approved |
| Water density | 1.025 (salt) / 1.000 (fresh) | tonnes per cubic metre | Standard hydrostatic values |
The governing identity is DWT = loaded displacement minus lightship. Each variable has a precise meaning:
- Loaded displacement is the total weight of the vessel when immersed to her summer load line. By Archimedes’ principle it equals the weight of the water displaced, computed as the moulded underwater volume multiplied by the density of the water the ship floats in.
- Lightship is the constant weight of the vessel as built and empty: the steel hull, the machinery, the permanent equipment and the fluids in the systems that cannot be pumped out. It does not change with loading and is established once by survey at delivery.
- Underwater volume is the volume of the hull below the waterline, read from the hydrostatic tables that the classification society approves against the hull lines plan.
- Water density converts that volume into a weight. Salt water is taken as 1.025 tonnes per cubic metre and fresh water as 1.000, so the same hull at the same draught displaces more weight in the sea than in a river.
Subtracting the fixed lightship from the loaded displacement leaves the variable weight the ship can take on: her deadweight. Summer deadweight is the figure quoted on the particulars, measured at the summer mark in salt water.
Deadweight vs cargo deadweight, displacement and tonnage
Deadweight is routinely confused with three adjacent measures. The distinctions are not pedantic: each describes a different thing, and a fixture priced against the wrong one is mispriced.
| Measure | What it measures | Units | Relation to DWT |
|---|---|---|---|
| Deadweight (DWT) | Total weight the ship can carry to her marks: cargo plus bunkers, water, stores, constants | tonnes (weight) | The reference figure |
| Cargo deadweight (DWCC) | Weight available for cargo alone, after deducting bunkers, fresh water, stores and constants | tonnes (weight) | DWCC = DWT minus bunkers, stores, water, constants |
| Displacement | Total weight of the ship and everything in her at a given draught | tonnes (weight) | Loaded displacement minus lightship = DWT |
| Gross / net tonnage | Internal volume of the ship (GT) and of her cargo spaces (NT) | dimensionless volume index | Unrelated dimension: volume, not weight |
The most consequential distinction for chartering is between deadweight and cargo deadweight (DWCC, deadweight cargo capacity). DWT is the whole carrying capacity. DWCC is what is left for cargo once the voyage consumables are loaded: bunker fuel for the passage, fresh water, lubricating oils, stores, spares and the constants (the small accumulated weights of mud, paint, spare gear and unpumpable residues that surveyors carry as a fixed allowance). On a long ballast-and-laden round voyage the bunker line alone can run to several thousand tonnes, so DWCC is always materially below DWT.
Deadweight against displacement is the same identity read the other way: displacement is the total floating weight, and deadweight is the part of it that is not lightship. Deadweight against tonnage is a category error people make constantly. Gross and net tonnage measure internal volume, not weight, and feed port dues, canal tolls and manning rules. A vessel has both a deadweight in tonnes and a gross tonnage as a volume index, and the two are not interchangeable. The tonnage page works through the volumetric measures in detail.
Where deadweight sits on the hull
Deadweight is the weight that takes a ship from her light waterline down to her loaded marks. Visually it is the layer of immersion between the two.
When the ship is empty she floats high, at her light waterline, displacing only her lightship weight. As cargo, bunkers and stores come aboard she sinks deeper and the underwater volume grows, displacing more water and therefore more weight. The summer load line, marked on the hull as the Plimsoll mark and its accompanying load-line disc, is the deepest she may legally float in salt water in normal conditions. The vertical band of hull between the light waterline and the summer waterline is the immersion that deadweight buys, which is why deadweight and draught move together and why a draught survey can be used to weigh the cargo loaded.
Worked deadweight calculation
Take a representative modern Panamax on a single coal voyage. The figures are representative round numbers, not a specific vessel.
| Step | Item | Figure | Running result |
|---|---|---|---|
| 1 | Loaded displacement at summer mark | 90,000 t | 90,000 t displacement |
| 2 | Less lightship | 12,000 t | DWT = 78,000 t |
| 3 | Less bunkers (HFO + MGO for the round voyage) | 2,200 t | 75,800 t |
| 4 | Less fresh water and stores | 600 t | 75,200 t |
| 5 | Less constants | 200 t | DWCC = 75,000 t |
Working through it: the ship’s loaded displacement at the summer load line is around 90,000 tonnes. Subtract her certified lightship of about 12,000 tonnes and the deadweight is 90,000 minus 12,000 = 78,000 tonnes. That 78,000 t is the summer deadweight, the headline DWT on her particulars.
To reach the cargo figure, deduct the voyage consumables from DWT. Bunkers for the laden and ballast legs come to roughly 2,200 t, fresh water and stores to around 600 t, and the surveyor’s constants to about 200 t. The cargo deadweight is therefore 78,000 minus 2,200 minus 600 minus 200 = 75,000 tonnes (DWCC). The charterer can lift 75,000 t of coal on this voyage even though the ship’s nameplate deadweight is 78,000 t, and the gap is entirely the weight of running the ship from load port to discharge port. Bunker the vessel for a longer voyage and the cargo intake falls further.
How deadweight varies by bulk carrier class
Deadweight is the primary axis along which the mainstream dry bulk fleet is divided into classes. Each class occupies a representative deadweight band, and the band drives draught, port access and the cargoes the class typically lifts.
| Class | Representative DWT band | Typical summer draught | Characteristic cargoes |
|---|---|---|---|
| Handysize | 10,000 to 40,000 t | ~10 m | Minor bulks, grain, steels, geared and flexible |
| Handymax / Supramax | 40,000 to 65,000 t | ~12.5 m | Grain, coal, minor bulks, geared |
| Supramax | 50,000 to 65,000 t | ~12.8 m | Grain, coal, fertilisers, geared |
| Panamax | 65,000 to 85,000 t | ~14 m | Coal, grain, bauxite, gearless |
| Capesize | 150,000 to 180,000 t | ~18 m | Iron ore, coal, bauxite, gearless |
Reading down the table, deadweight rises with the size of the hull and pulls draught up with it, which is what closes ports to the larger classes. A Handysize at 10 m summer draught can enter the great majority of cargo ports; a Capesize at 18 m can only call at the handful of deepwater iron ore and coal terminals built for her. The Handymax and Supramax bands overlap, the distinction being generation and gear rather than a clean deadweight break. The bulk-carriers hub carries the full class catalogue and the size-comparison page sets the bands side by side. Treat every band here as representative: published class definitions vary at the edges and the figures should be checked against current Clarksons fleet data before use.
Salt water, fresh water and other edge cases
Deadweight is quoted at the summer mark in salt water, but the achievable deadweight at a given draught changes with the water the ship floats in, and that catches operators out.
The mechanism is density. Loaded displacement equals underwater volume times water density, and salt water at 1.025 tonnes per cubic metre is denser than fresh water at 1.000. A ship loaded to her marks in the sea will float roughly 2.5 percent deeper if she then moves into fresh water, because the same weight needs more volume, hence more draught, to stay afloat in the lighter water. This is why ships carry a fresh water allowance (FWA) and a dock water allowance (DWA): the load line includes a fresh-water mark that sits above the salt-water summer mark, permitting the hull to be loaded deeper in a river or dock so that she rises to her correct summer marks once she reaches the open sea. Load a ship to her salt-water summer mark while she sits in fresh water and she will be overloaded when she puts to sea.
Two further constraints cap maximum deadweight in practice:
- Seasonal load lines. The Load Lines Convention assigns marks for summer, winter, tropical and other zones. Winter and Winter North Atlantic marks sit below the summer mark, so the lawful deadweight in a winter zone is less than the nameplate summer deadweight. A voyage routed through a winter zone is capped by the lower mark, not the summer figure. The interaction is governed through the load line marks on the hull.
- The binding constraint is rarely volume. For dense cargoes such as iron ore, the ship reaches her deadweight and draught limit long before the holds fill, so deadweight is the cap. For light cargoes such as grain at full stow, the holds fill first and cubic capacity binds before deadweight does. Which limit binds determines whether a fixture is “deadweight-limited” or “volume-limited”, and it changes the freight-per-tonne arithmetic.
These are not arithmetic adjustments to the formula so much as constraints on which deadweight figure applies, but they change the tonnage the ship can actually lift, so they belong in any competent intake calculation.
Scope and what this page does not cover
This page explains deadweight tonnage as a measure of a vessel’s carrying capacity: what it means, how it is derived from displacement and lightship, how it differs from cargo deadweight, displacement and volumetric tonnage, and how water density and seasonal marks shift it. It does not perform a vessel-specific draught survey, certify lightship or deadweight for a named ship, interpret a particular load-line certificate, or replace the approved hydrostatic tables and stability booklet that govern an actual loading. Those are matters for the master, the classification society and a qualified draught surveyor, working from the vessel’s own documents. The figures here are representative, not measurements of any specific vessel.