Tonnage

What a ship can actually lift

Published 23 August 2026 · updated 17 October 2026

A charterer wants 55,000 tonnes of iron ore. An owner offers a Supramax of 58,000 deadweight. Deadweight is the total weight a ship can carry. The numbers look like a match, but deadweight is not cargo, and the port is usually shallower than the ship needs. To work out what the ship can really lift, you make four subtractions and one comparison. The answer is often thousands of tonnes below the figure on the circular (the list of ships an owner or broker sends out).

The arithmetic, written out

Supramax, iron ore. Illustrative figures, consistent with each other
and not a description of any particular ship.

  Deadweight at the summer marks                     58,000
  Less bunkers for the voyage                         1,400
  Less fresh water                                      200
  Less constants                                        290
                                                   --------
  Deadweight cargo capacity                          56,110

  The ship's summer marks                             12.80 m
  The load berth can work                             12.00 m
  Short by                                             0.80 m = 80 cm
  TPC 52.8, so 80 cm of draft is                      4,224 mt

  Cargo the ship can lift THERE                      51,886

  Wanted                                             55,000
  Short by                                            3,114

None of this is hard. But it needs four numbers that are not on the circular, and one comparison the circular cannot make. That comparison decides whether there is a deal.

Deadweight is a total, not a cargo allowance

Deadweight is everything the ship can carry when it is loaded down to its marks (the load line painted on the hull). That means cargo, and also fuel, water, stores, spare parts and crew. Three things come off it before any cargo goes on.

• Bunkers (the ship's fuel). The amount depends on the voyage, not the ship. An ocean voyage carries well over a thousand tonnes of fuel, a coastal trip a small part of that. The voyage estimate already contains this figure, so work out the estimate and the intake together.
• Fresh water. A couple of hundred tonnes. It is small, but it is still freight.
• Constants. Everything on board that is not fuel, water or cargo: stores, spare parts, paint, sludge, mud in the ballast tanks, water that cannot be pumped out, the crew and their belongings. On a bulk carrier this is a few hundred tonnes.

Deadweight minus those three is the figure a broker wants. It has its own name: deadweight cargo capacity. People argue about constants, because nobody measures them directly. The constant is what is left when you subtract the lightship (the empty ship's weight) and the cargo from the displacement (the total weight of the ship and everything in it). So it hides all the error of the draft survey. A careful survey is accurate to about half to one per cent. On a Panamax cargo, that is more tonnes of measurement error than the whole constant. So if the constant is very different at load and at discharge, the ship almost never gained weight: one of the two surveys is wrong.

Three limits, and the answer is the lowest

Exactly three things can stop cargo going on board, and every intake question is about which one applies first.

• Weight. Deadweight cargo capacity, as above. Everyone quotes this limit, but often it is not the one that decides.
• Draft. How deep the ship can sit in the water in the shallowest place on the voyage. This limit decides most part cargoes, and you cannot see it in a deadweight figure.
• Volume. Whether the holds fill up first, which decides every light cargo.

Draft, and the number that converts it into tonnes

If a port can work 12.0 metres, it does not matter that the ship is marked for 12.8. Every centimetre the ship cannot use costs it cargo. The conversion rate is TPC, tonnes per centimetre immersion. TPC is the weight needed to push the ship one centimetre deeper.

TPC is a property of the hull at a given draft, shown on the ship's capacity plan. It depends on the area of the hull at the waterline. Without the plan, you can estimate it from length and breadth to within a few per cent. That is good enough to decide whether to offer the ship, but not to declare a quantity. TPC also changes with draft, so read it at the draft you are using.

On a Supramax, TPC is in the low fifties. So one metre of draft restriction is over five thousand tonnes of cargo. No rate negotiation on a Supramax cargo moves five thousand tonnes. A broker who checks the deadweight and not the berth has checked the less important number.

The season moves the mark itself

A ship's load line is not one line. It has summer, winter and tropical marks. The convention that sets them uses simple arithmetic. The winter mark is below the summer mark by one forty-eighth of the summer draft. The tropical mark is the same distance above it. On the Supramax above, one forty-eighth of 12.8 metres is close to 27 centimetres. At its TPC, that is about fourteen hundred tonnes of cargo the ship can carry in July and cannot carry in January.

The load line zones and seasonal periods decide which mark applies, based on where the ship is and the month. The North Atlantic in winter is the common case. It is a fact about the calendar and the map, not the ship. It changes the intake more than most rate arguments, and no deadweight figure shows it. Brokers who trade there in winter know it. Others often forget it.

Fresh water gives back some draft

Load lines are set for salt water. In fresher water a ship floats deeper for the same weight. So it may load past its marks by the difference. This is the fresh water allowance, adjusted for how far the dock water is from full salt water. At a river berth this is not a small amount. On the ship above it is around a third of a metre in fully fresh water. That is over a thousand tonnes of cargo.

The allowance is not extra cargo. Loading to the fresh water mark in a river brings the ship to the same deadweight it would reach on its summer mark at sea. If a broker knows the load berth is in fresh water and does not use the allowance, that cargo is not loaded.

Volume, and cargoes that cube out

The third limit has nothing to do with weight. Every cargo takes up a certain volume per tonne. This is its stowage factor. Divide the ship's hold capacity by it, and you get the number of tonnes that will physically fit.

When a cargo fills the holds before the ship reaches its marks, the ship has cubed out. When a cargo brings the ship to its marks with the holds part empty, the ship has deadweighted out. Ore does the second, and grain usually does the first.

Take the worked example's Supramax at a berth with no depth limit, with the normal grain capacity for its class, about 71,000 cubic metres. With soyabeans at 1.35, the holds take about 52,600 tonnes. So the ship cubes out with thousands of tonnes of deadweight it cannot use. With iron ore, it reaches its marks with the holds only about a third full. Only the cargo changed.

Three details have each cost someone a cargo.

• Grain capacity is not bale capacity. Grain capacity is measured to the full inside limits of the hold, out to the frames and up into the hatch coamings (the raised edges around the hatch). It is the space a cargo that flows into every corner can use. Bale capacity is measured to the inside of the frames or cargo battens. It is the space a bagged, baled or packed cargo can use. The usual gap is seven to ten per cent. That figure comes from older ships with frames and several decks. On a modern bulk carrier with smooth sides the gap is much smaller. Use the wrong one and your answer is wrong by the difference.
• Cubic feet are not all the same. Half the market still gives stowage in cubic feet, and the traditional quote is per long ton, not per tonne. One conversion is about 35.3 and the other about 35.9. Choosing the wrong one moves a Supramax cargo by hundreds of tonnes. Nobody notices until the ship is at the berth.
• The published figure is a starting point, not a fact about this cargo. The figure the shipper declares is the one that applies. Moisture changes it. The same cargo from two different ports has a different number.

The one number to keep for each ship

Divide the ship's grain capacity by its deadweight cargo capacity. The answer is the stowage factor at which the ship cubes out and deadweights out at the same time. On the Supramax above it is about 1.27. Any cargo lighter than that fills the holds first. Any cargo heavier than that brings the ship to its marks first. It is a fixed property of the hull, and it answers the volume question for every cargo the ship is offered.

This number also gets smaller as ships get bigger. A geared Supramax has more hold space per tonne of deadweight than a Capesize. That is the main reason grain moves on Supramaxes and Panamaxes and iron ore moves on Capesizes. It is also why coal is the cargo that causes arguments. Coal is quoted at anything from around 1.1 from Australia to over 1.5 from West Africa. That range sits on both sides of this number. So the same ship deadweights out on coal from one place and cubes out on coal from another. Never plan a coal cargo on a general figure.

Which port governs

The limit that applies is the shallowest water the ship has to work in. That is not always the load port. A broker who checked only the load port can fix a cargo the ship cannot deliver. Tides make it harder. A berth may reach its stated depth only for part of the day. A ship waiting for enough water is a ship using laytime.

The Parana river is the clearest example in the trade. On a given day, the up-river berths at Rosario and San Lorenzo can load a ship deeper than the falling river will let it leave. Traders use that gap. It is a normal way of working, not only a problem. The ship part-loads up river and finishes at a deeper port downstream. On the way it gains the extra draft and the rise from fresh water into salt water. At the discharge end the same thing happens in reverse: cargo is moved off a deeply loaded ship into smaller vessels (lightering) so that it can enter port. The Mississippi below New Orleans and the tidal iron ore berths of north-west Australia are the other two common cases. In Australia, the draft a ship may sail with depends on the tide on the day, not on any published figure. All of this costs real money and time. It must be in the estimate, not in a footnote.

The number in the charterparty is not a single number

Nobody can know the exact intake in advance. So the charterparty fixes the quantity with a tolerance (a percentage either side of the stated figure) and an option: one named party declares the final figure before loading. On a dry cargo enquiry this is MOLOO, more or less in owner's option, or MOLCHOPT when the charterer declares. The sides negotiate which applies, because the party that declares controls how much freight the tolerance is worth. Behind it is the duty to load a full and complete cargo. Freight is earned on the quantity actually loaded, and deadfreight is owed on any shortfall the charterer failed to supply. These rules exist because a single number would be a promise nobody could keep.

None of this is advanced: four subtractions, one multiplication and choosing the smallest of three numbers. It is worth doing properly because it comes at the start of a negotiation. The person who has done it knows whether the enquiry on screen is a real cargo for this ship or a waste of time.

References

• Britannia P&I, Draft survey calculation on a ship: practical guidance, 2023. britanniapandi.com
• IMO, International Convention on Load Lines, 1966. imo.org
• IMO, International Convention on Tonnage Measurement of Ships, 1969. imo.org
• IMO, International Maritime Solid Bulk Cargoes (IMSBC) Code. imo.org

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