Insights · Specs · Tonnage & load lines
Two of the oldest numbers in shipping decide a vessel's size and its safe limit: tonnage, which measures its volume, and the load line, which fixes how deep it may legally sit in the water. Neither is what its name suggests — and both are set by international convention.
If you read a ship's specs and assume "tonnage" means weight, you will misread almost every figure that follows. Tonnage, in the maritime sense, is mostly about volume and the legal categories that flow from it. And the famous Plimsoll line painted on a hull is not a size at all but a safety limit. Two separate conventions sit behind these numbers, and understanding the difference is the foundation of reading a vessel correctly.
Before 1969, different countries measured ships in different ways, so the same hull could carry different "tonnage" depending on where it was registered. The International Convention on Tonnage Measurement of Ships, 1969 (ITC 1969) fixed that. Adopted in 1969 and in force from 18 July 1982, it replaced the old gross and net register tonnage with two new figures calculated by a single global formula:
Because they are dimensionless, GT and NT are not measured in tonnes. Their job is to set thresholds: manning levels, which safety rules apply, registration fees, and the port dues a ship pays. This is exactly why GT must not be confused with deadweight (DWT), which is a weight — the cargo, fuel and stores a ship can carry. We separate the two in detail in gross tonnage vs deadweight.
| Measure | What it measures | Unit |
|---|---|---|
| Gross tonnage (GT) | Total enclosed volume | dimensionless |
| Net tonnage (NT) | Cargo / earning volume | dimensionless |
| Deadweight (DWT) | Carrying capacity by weight | tonnes |
The second number is a safety limit with a human story behind it. In the nineteenth century, owners routinely overloaded ships — sometimes deliberately over-insured "coffin ships" — sending crews to sea in vessels riding dangerously low. The reformer Samuel Plimsoll campaigned in the British Parliament until a mandatory load-line mark was imposed. That mark still bears his name.
The load line is a circle crossed by a horizontal line, painted amidships on both sides of the hull, showing the maximum depth to which the ship may be loaded. Around it sit further marks for different conditions, because a ship floats higher in dense cold sea water than in warm fresh river water:
The load line is governed by the International Convention on Load Lines, 1966, which entered into force in 1968. The freeboard it fixes — the distance from the waterline to the deck — is what guarantees a margin of buoyancy and reserve stability. Load a ship past its mark and that margin disappears.
The Plimsoll line is the rare piece of maritime regulation a layperson can read with their own eyes: if the water is above the mark, the ship is loaded beyond what the law allows. On the International Convention on Load Lines, 1966
Tonnage and load line together frame almost everything else on a ship's file. GT decides which SOLAS rules apply and what the ship pays in port; NT and DWT describe what it can earn and carry; the load line sets the limit it must not cross. A vessel's reported GT, NT, DWT and draught are the headline figures in every registry entry. On Marifest you can search any of 97,000+ vessels in the registry, read those measurements alongside its type, builder and flag, and check its compliance standing in one view. Every term used here is defined in the maritime glossary.
How Marifest uses it
GT, NT, DWT and draught each answer a different question. Marifest keeps them distinct on every vessel file, so the size of a ship is never mistaken for its weight.
Gross and net tonnage sit beside deadweight on every file — clearly distinguished, so the dimensionless figures are never confused with the weight.
A ship's gross tonnage decides which SOLAS and other thresholds apply — and the GT is on the record for every vessel.
Reported draught, the figure the load line constrains, is part of the specs surfaced for each ship.
Size, type, builder, flag and compliance standing sit together, so the headline numbers are always read in context.