The word “ton” in a maritime context is genuinely ambiguous unless you know which specific meaning applies — shipping uses it in several distinct ways.
Metric Ton (Weight)
A metric ton equals 1,000 kilograms. This is a straightforward weight unit, used for cargo weight, fuel quantities, and similar mass measurements — the sense of “ton” most people are already familiar with from everyday use.
Tonnage Measurements: A Different Concept Entirely
Where things get genuinely confusing is that a ship’s size and capacity are also described using “tonnage” — but these are not weight measurements in the everyday sense:
- Gross Tonnage (GT) — measures total enclosed volume, not weight
- Net Tonnage (NT) — measures the volume of revenue-earning (cargo) spaces
- Deadweight Tonnage (DWT) — this one is a weight measurement — the maximum weight (cargo, fuel, stores, crew) a vessel can carry
This is why a ship described as “180,000 DWT” is being described by weight capacity, while the same ship’s “GT” figure describes its volume — two completely different numbers, both called “tonnage,” measuring different things.
Why This Matters Practically
Regulatory thresholds (SOLAS, MARPOL, STCW requirements) are often written in terms of GT, while commercial chartering and cargo capacity discussions use DWT — confusing the two when reading a regulation or a charter party is a genuine, avoidable error.
Long Ton and Short Ton — Older Imperial Variants
Some older or region-specific references may use the long ton (2,240 lbs, ~1,016 kg) or short ton (2,000 lbs, ~907 kg) rather than the metric ton — worth knowing these exist, though modern international shipping has largely standardized on metric tons and the tonnage measures above.
Exam Relevance
Correctly distinguishing weight-based “ton” from volume-based GT/NT and weight-based DWT is a foundational Ship Knowledge topic tested in deck and engineering officer competency exams — see the separate entries on Gross Tonnage, Net Tonnage, and Deadweight Tonnage for the detailed calculations.