Measurement

How a ship is measured

Published 28 August 2026 · updated 22 September 2026

A ship carries several numbers that all sound like size, and they come from different measurement systems written for different purposes in different centuries. Two of the most quoted are not weights at all. Knowing which system produced a figure tells you what the figure is allowed to decide.

Tonnage is a volume, and it is not tonnes

The word tonnage descends from the tun, a cask of wine, and it has measured internal capacity ever since. Under the International Convention on Tonnage Measurement of Ships, 1969, which came into force in 1982, gross tonnage is derived from the total enclosed volume of the ship by the formula GT = K₁V, where V is that volume in cubic metres and K₁ is 0.2 + 0.02 log₁₀V.

Read that carefully. GT is a dimensionless index, not a weight and not a volume either, because the coefficient bends with size. Net tonnage is a second formula over the cargo spaces alone, with the convention setting a floor: net tonnage may not be taken as less than 30 per cent of gross tonnage.

The 1969 convention replaced the older register ton, a unit of 100 cubic feet, and the terms that went with it. You will still meet GRT and NRT on databases and older documents, and the safest reading is that a modern figure is convention tonnage and an old one may not be. They are not interchangeable and there is no conversion factor.

The three weights

Displacement is the weight of water the hull pushes aside, and by Archimedes it is the weight of the ship and everything in her. It is the only genuine weight in the set. Lightweight, usually quoted as LDT, is the ship herself with nothing aboard, and it is the figure a demolition sale is priced on. Deadweight is the difference between the two at a stated draught: everything she can carry, which means cargo plus bunkers, fresh water, stores, crew and ballast.

Deadweight is therefore a capacity, not a cargo figure, and the gap between summer deadweight and what a ship will actually lift on a given voyage is where a great deal of money is lost. That arithmetic is its own subject.

Figure	What it measures	What it decides
Gross tonnage	Enclosed volume, by formula	Manning, survey scope, safety rules, most port dues
Net tonnage	Cargo space volume, by formula	Dues and charges meant to fall on earning capacity
Displacement	Weight of water displaced	Stability, and the whole of a draft survey
Lightweight	The empty ship	Scrap value
Deadweight	Carrying capacity at a draught	Size class, and the ceiling on a cargo
Draught	Depth of hull below the waterline	Which berths, rivers and channels she can reach

Freeboard: the mark that says how deep she may go

Deadweight only means something at a stated draught, and the maximum draught is not the owner’s choice. The International Convention on Load Lines, 1966 assigns a freeboard, the distance from the waterline to the freeboard deck, and marks it on the side of the hull. The load line disc and the ladder of marks beside it set different limits for different water and different seasons: tropical, summer, winter, and winter North Atlantic, with fresh water allowances alongside.

The seasonal marks are why a ship’s deadweight changes with the calendar and the ocean. Winter North Atlantic is a real reduction in what she may load, and a laycan that straddles a seasonal boundary is a quantity question as well as a scheduling one. Fresh water is less dense than salt, so a ship floats deeper in a river and rises as she reaches the sea, which the fresh water allowance is there to permit.

Length, breadth and the rest of the geometry

LOA, length overall, is the figure a berth cares about. LBP, length between perpendiculars, is shorter and is the figure naval architecture and hydrostatic tables use. Beam is maximum breadth, moulded to the frames. Depth is a structural dimension from keel to freeboard deck and is not draught. Air draught is the height from the waterline to the highest point, which is what a bridge or a loading gantry cares about, and it varies with how deeply she is loaded.

The canals keep their own books

Where a toll is charged on capacity, the authority charging it does not accept somebody else’s measurement. The Panama Canal measures ships under the PC/UMS system, set out in its own admeasurement regulations and based on the 1969 convention rules but computed by the canal for tolls. The Suez Canal maintains Suez Canal Net Tonnage, a separate certificate issued to the canal’s own rules, and a ship transiting without a valid one is charged provisionally on the canal’s gross figure instead.

So a single ship can carry an international gross tonnage, an international net tonnage, a Panama tonnage and a Suez tonnage, all different, all correct, and none convertible into the others. That is not a defect in the system. It is what happens when four bodies with different interests each measure the same steel.

The trap

The trap is treating any of these as a general purpose measure of how big a ship is. They are answers to specific questions. Deadweight answers what she lifts, gross tonnage answers what she costs in dues, draught answers where she can go, and cubic capacity answers whether a light cargo will fit before her deadweight is used up. Two ships of identical deadweight can have materially different gross tonnages, and therefore different port costs on the same voyage, which is a difference in a voyage estimate rather than a curiosity.

References

IMO, International Convention on Tonnage Measurement of Ships, 1969. imo.org
IMO, International Convention on Load Lines, 1966. imo.org
Panama Canal Authority, Regulations for the Admeasurement of Vessels for the Panama Canal, 2019. pancanal.com
Suez Canal Authority, Rules of Navigation. suezcanal.gov.eg
IMO, International Convention for the Safety of Life at Sea (SOLAS), 1974. imo.org

Reading a ship's particularsWhat each figure on a vessel description constrains, why GRT is a volume, and why draught excludes a ship long before deadweight does. What a ship can actually liftA 58,000 deadweight ship does not lift 58,000 tonnes, and at a shallow berth she may not lift 52,000. The arithmetic nobody does in their head. Dry bulk vessel sizesHandysize to Capesize: where the deadweight bands sit, why the boundaries are conventions, and how many hulls are in each. Hull form and resistanceWhat a ship burns is set by the shape she drags through the water, and by how clean it has been kept since the last drydocking.

Neptune Atlas

Neptune Atlas holds 88,634 hulls with a GRT and 75,596 with a deadweight, and shows which of the two a result was matched on. A figure that is absent is reported as absent rather than left out of the count, because port dues and lift capacity are not the same question. Every paid plan starts with 7 free days. A card is needed to start them, and cancelling before they end costs nothing.

Try 7 days free Ship chartering software for brokers What each plan includes