Gross tonnage (GT) and net tonnage (NT) are both volume-based measurements of a ship, calculated under the International Convention on Tonnage Measurement — despite the word “tonnage,” neither measures weight.
Gross Tonnage (GT)
GT measures the ship’s total enclosed volume — essentially the entire internal space of the vessel, calculated using a formula based on the ship’s total enclosed volume in cubic meters.
Net Tonnage (NT)
NT measures specifically the volume of the ship’s cargo-carrying (revenue-earning) spaces, excluding spaces like machinery rooms, crew accommodation, and other non-cargo areas — reflecting the vessel’s actual earning capacity rather than its total size.
Why the Distinction Matters
- GT is commonly used as the threshold for regulatory requirements — many SOLAS, MARPOL, and STCW requirements apply based on GT thresholds (for example, AIS carriage requirements reference GT)
- NT is often used as the basis for port dues, canal transit fees, and other charges tied more directly to a vessel’s commercial cargo capacity than its overall size
Neither Measures Weight
A common point of confusion: despite “tonnage” in the name, GT and NT are volume measurements, not weight. Deadweight tonnage (DWT), by contrast, does measure a weight-based capacity — the maximum weight a vessel can carry.
Exam Relevance
Gross and net tonnage definitions, calculation basis, and their regulatory/commercial applications are standard Ship Knowledge topics in deck officer competency exams.