Measurement

How a ship is measured

Published 28 August 2026 · updated 17 October 2026

A ship has several numbers that all sound like its size. They come from different measurement systems, written for different purposes in different centuries, and two of the most quoted are not weights at all. Knowing which system produced a figure tells you what that figure can be used to decide.

Tonnage is a volume, and it is not tonnes

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

Note what this means. GT is an index without units. It is not a weight, and it is not quite a volume either, because the multiplier changes with the size of the ship. Net tonnage is a second formula applied only to the cargo spaces, and the convention sets a minimum: net tonnage may not be 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 see GRT (gross register tons) and NRT (net register tons) in databases and older documents. The safest reading is that a modern figure is convention tonnage and an old one may not be. The two are not interchangeable, and there is no factor for converting one into the other.

The three weights

Displacement is the weight of the water the hull pushes aside. By Archimedes' principle, it equals the weight of the ship and everything on it, and it is the only true weight in the set. Lightweight, usually quoted as LDT, is the weight of the empty ship with nothing on board, and it is the figure a sale for scrap is priced on. Deadweight is the difference between the two at a stated draught. It is everything the ship can carry: cargo plus fuel, fresh water, stores, crew and ballast.

So deadweight is a capacity, not a cargo figure. A lot of money is lost through the difference between summer deadweight and the cargo a ship can really carry on a given voyage. That calculation is a subject of its own.

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

Freeboard: the mark that says how deep the ship may go

Deadweight only has a meaning at a stated draught, and the owner does not choose the maximum draught. The International Convention on Load Lines, 1966, sets a freeboard, which is the distance from the waterline up to the freeboard deck, and it is marked on the side of the hull. The load line disc and the row of marks next to it set different limits for different waters and seasons: tropical, summer, winter, and winter North Atlantic, with allowances for fresh water as well.

These seasonal marks are why a ship's deadweight changes with the time of year and the ocean. In the winter North Atlantic, a ship may load noticeably less. A laycan that crosses a seasonal boundary therefore raises a question about quantity as well as timing. Fresh water is less dense than salt water, so a ship floats deeper in a river and rises as it reaches the sea. The fresh water allowance exists to permit this.

Length, breadth and the rest of the geometry

LOA, length overall, is the figure a berth needs to know. LBP, length between perpendiculars, is shorter, and naval architects and the hydrostatic tables use it. Beam is the maximum width, measured to the inside of the frames. Depth is a structural measurement from the keel to the freeboard deck, and it is not the same as draught. Air draught is the height from the waterline to the highest point of the ship, which matters for bridges and loading gantries, and it changes with how deeply the ship is loaded.

The canals keep their own books

When a toll is charged on a ship's capacity, the authority charging it does not accept anyone else's measurement. The Panama Canal measures ships under its PC/UMS system, set out in its own measurement regulations. The system is based on the 1969 convention rules, but the canal does its own calculation for tolls. The Suez Canal keeps the Suez Canal Net Tonnage, a separate certificate issued under the canal's own rules. A ship that passes through without a valid certificate is charged on a provisional basis, using the canal's gross figure instead.

So one ship can have an international gross tonnage, an international net tonnage, a Panama tonnage and a Suez tonnage. All four are different, all are correct, and none can be converted into the others. This is not a fault in the system. It is what happens when four bodies with different interests each measure the same ship.

The trap

The trap is using any one of these figures as a general measure of how big a ship is. Each one answers a specific question. Deadweight tells you what the ship can carry, gross tonnage tells you what it pays in dues, draught tells you where it can go, and cubic capacity tells you whether a light cargo will fit before the deadweight is used up. Two ships with the same deadweight can have clearly different gross tonnages, and so different port costs on the same voyage. That difference belongs in the voyage estimate.

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 limits, why GRT measures volume, and why draught rules a ship out 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 it may not lift 52,000. The sums, written out step by step. Dry bulk vessel sizesHandysize to Capesize: where the deadweight bands sit, why their edges are trade habits, and how many ships fall in each. Hull form and resistanceHow much fuel a ship burns depends on the shape of its hull, and on how clean that hull has been kept since the last drydocking.

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