A ship described as 20,000 GT does not necessarily weigh 20,000 tonnes. The abbreviation belongs to a measurement system based on enclosed volume. That distinction matters when comparing a ferry, a cargo vessel and a historic ship whose record uses older terminology.
Start with the label beside the number
The IMO explanation of tonnage measurement identifies gross tonnage as a function of the moulded volume of a ship’s enclosed spaces. It also distinguishes modern GT and NT from the older gross and net register tonnage terms. Copy the label exactly before trying to compare two records.
GT is not simply the internal volume written in cubic metres: the convention uses a calculation. Nor should you replace it with a cargo mass. A passenger ship can have extensive enclosed accommodation that serves a different purpose from a freighter’s holds.
Make a small comparison card
For each vessel, record its name, the stated measurement, the unit or abbreviation, the source and the record date. Leave separate rows for displacement and carrying capacity if the source supplies them. An empty row is more useful than a guessed conversion.
Suppose a museum panel gives an older ship’s register tonnage while a modern operator lists GT. Your first finding is that the records use different measures. Finding comparable documentation is the next task; dividing the two numbers immediately would create an impressive-looking ratio without a dependable meaning.
AI-generated editorial illustration; not a documentary photograph.