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Gross Register Tonnage (GRT): Measures the vessel’s total internal volume (think engine rooms, cargo holds, living spaces) and influences port fees, safety rules, and regulations.

Net Register Tonnage (NRT): Focuses on cargo-carrying spaces, representing the vessel’s earning capacity.

Deadweight Tonnage (DWT): The total weight a vessel can safely carry, including cargo, fuel, and provisions. A critical metric for capacity planning.

Displacement Tonnage: The total weight of the vessel and its contents—commonly used for naval vessels and stability analysis.

Lightweight Tonnage (LWT): The vessel’s weight without fuel or cargo—important for ship valuation and scrapping.

Cargo Tonnage: Represents the actual weight or volume of cargo carried.

Special Canal Tonnage: Used specifically to calculate transit fees for the Panama and Suez Canals.
Why it matters:
Tonnage isn’t just a technical metric—it’s the foundation of operations, commercial decisions, and regulatory compliance in the shipping industry. From port fees to international safety conventions, tonnage affects it all.