Gross & Net Tonnage

What is tonnage in shipping?

Tonnage is a volumetric measure of a ship, not a weight. Under the International Convention on Tonnage Measurement of Ships, 1969 (ITC 69), gross tonnage (GT) and net tonnage (NT) are dimensionless index numbers derived from a vessel’s enclosed volume. They are not counts of tonnes.

The word “tonnage” causes more confusion in shipping than almost any other technical term, because it sounds like it should describe how heavy a ship is or how much cargo she can lift. It does neither. GT and NT are derived from how much enclosed space a vessel encloses, measured in cubic metres and then run through a logarithmic formula. The weight measures, deadweight and displacement, are entirely separate quantities expressed in tonnes. Confusing the two is the single most common error when a charterer, port agent or student reads a vessel’s particulars.

The legal anchor for modern tonnage is the International Convention on Tonnage Measurement of Ships, 1969 (ITC 69), administered by the International Maritime Organization (IMO). ITC 69 came into force in 1982 and replaced the older national systems, including the long-standing Moorsom and British Register Ton conventions, with a single international method. Under that method GT captures the total enclosed volume of the ship and NT captures the useful, earning volume. Both figures appear on a vessel’s International Tonnage Certificate, issued by or on behalf of the flag state, and both are recorded in the specifications the bulk-carriers hub catalogues for each class.

Gross, net, deadweight and displacement compared

The four headline numbers on a bulk carrier’s particulars are easy to mix up because two are volumes and two are weights. The table below sets them side by side. It is the contrast that the rest of this page builds on: GT and NT measure space, while deadweight and displacement measure mass.

Measure	What it expresses	Formula / basis	Unit	Reference
Gross tonnage (GT)	Total enclosed volume of the ship, as an index	GT = K1 × V, where K1 = 0.2 + 0.02·log10 V and V = total enclosed volume in m3	Dimensionless index	ITC 69, Annex I, Regulation 3 (IMO)
Net tonnage (NT)	Useful, earning volume (cargo spaces, plus a passenger term)	Derived from moulded cargo-space volume with draught and depth factors, plus a passenger component	Dimensionless index	ITC 69, Annex I, Regulation 4 (IMO)
Deadweight (DWT)	Weight a ship can carry: cargo, bunkers, stores, water, crew	Loaded displacement minus lightship weight	Tonnes	Class register / vessel particulars
Displacement	Weight of water the ship displaces, equal to her own weight	Volume of displaced water × water density	Tonnes	Class register / vessel particulars

The constants in the GT formula come straight from ITC 69, Annex I, Regulation 3: K1 is a coefficient that scales with the logarithm of enclosed volume, so a larger ship has a slightly higher multiplier applied to her volume. The K1 expression and the regulation references above should be checked against the convention text before publication. The practical takeaway is that GT and NT carry no unit. A bulk carrier described as “30,000 GT” does not weigh 30,000 tonnes and cannot carry 30,000 tonnes. Her deadweight and her displacement are different numbers entirely, both measured in tonnes.

Gross tonnage vs net tonnage

Both gross and net tonnage exist because they answer different regulatory and commercial questions, and a ship needs both figures on her certificate. They are derived from the same hull but capture different volumes.

Gross tonnage is the broad measure. It represents the total enclosed volume of the vessel, including the hull, the superstructure, and most enclosed spaces above and below the main deck. Because it scales with the overall size of the ship, GT is the figure that triggers most regulatory thresholds. Manning scales, survey scope, the requirement to carry certain equipment, the application of SOLAS and MARPOL provisions, and pilotage and safety regimes are commonly keyed to GT bands. When a regulation says it applies to ships “of 500 gross tonnage and above” or “of 3,000 gross tonnage and above”, it is reading off GT.

Net tonnage is the narrower measure. It is derived from the volume of the cargo-carrying spaces, adjusted by factors for moulded draught and depth, plus a term for passenger spaces on ships that carry passengers. NT is intended to reflect the earning capacity of the vessel, the volume she can actually monetise. Because of that, NT is the figure that port and harbour dues are most often levied on, and it feeds the canal tolling systems. A ship with large machinery, ballast or accommodation spaces relative to her cargo holds will show a lower NT-to-GT ratio. For a gearless bulk carrier with large, clear holds, NT is a high fraction of GT, which reflects how much of the ship is given over to revenue cargo space.

The measured spaces

Tonnage is the loosest visual fit of any vessel attribute, because it is an abstract volume rather than a dimension you can lay a tape measure against. The placeholder below shows the enclosed and measured spaces that feed the calculation: the moulded hull volume below the main deck and the enclosed superstructure and deckhouse volumes above it. GT counts essentially all of these enclosed spaces; NT counts the cargo-carrying subset, adjusted by the draught and depth factors.

Image Placeholder Enclosed and measured volumes of a bulk carrier feeding the GT and NT calculation Sectional view of a gearless bulk carrier shading the enclosed volumes counted for tonnage: the moulded hull below the main deck and the enclosed deckhouse and superstructure above. GT sums all enclosed spaces; NT counts the cargo-hold volume adjusted by draught and depth factors under ITC 69.

In practice the measurement is carried out by the classification society acting on behalf of the flag state, working from the approved drawings rather than from a physical survey of every compartment. The enclosed volume V that drives the GT formula is the sum of all enclosed spaces measured to the inside of the framing, in cubic metres. Spaces that are open to the sea or to the weather, and certain exempted spaces defined in ITC 69, are excluded. The resulting GT and NT are then certified on the International Tonnage Certificate and do not change unless the vessel is structurally altered.

Why tonnage matters: dues, regulation and canal tolls

Tonnage is not an academic figure. It is the basis on which a great deal of the money in a vessel’s operating account is charged. The main reasons the desk cares about a ship’s GT and NT:

• Port and harbour dues: most ports levy entry, berth and harbour dues on a per-GT or per-NT basis. NT is the more common base for cargo-related dues because it tracks earning capacity, while GT is often used for safety and pilotage charges. The exact base varies by port authority and should be checked against the local tariff.
• Manning and safety regulation: minimum safe manning, survey scope, and the application of many SOLAS and MARPOL requirements are keyed to GT bands. A ship crossing a GT threshold can move into a stricter regulatory category.
• Suez Canal: the Suez Canal Authority levies transit tolls on a dedicated measure, the Suez Canal Net Tonnage (SCNT), certified on a separate Suez Canal Special Tonnage Certificate. SCNT is its own measurement system and is not the same number as the ITC 69 net tonnage.
• Panama Canal: the Panama Canal Authority uses the Panama Canal Universal Measurement System (PC/UMS), which produces a PC/UMS net tonnage used to assess transit tolls. Like SCNT, this is a distinct system from ITC 69 and yields a different figure.
• Registration and class fees: flag-state registration fees, tonnage tax regimes in several jurisdictions, and classification society fees are frequently scaled on GT or NT.

The headline point is that ITC 69 GT and NT, Suez Canal Net Tonnage, and PC/UMS net tonnage are three separate measurement systems. A single bulk carrier carries different tonnage figures for each, and a voyage estimate that routes through a canal must pick up the right one. The canal-system details above should be verified against the current Suez Canal Authority and Panama Canal Authority tonnage rules before publication.

How tonnage varies by bulk carrier class

GT and NT rise with the size of the vessel, so each bulk carrier class sits in a characteristic tonnage band. The figures below are representative ranges for modern, gearless tonnage in each class. Click through to each class page for the full dimensional and chartering profile. The values are indicative and should be checked against classification society and Clarksons fleet data before publication.

The pattern to read off the table is that NT runs at roughly half to two thirds of GT for a typical gearless bulk carrier, because the cargo holds are the dominant enclosed volume on a ship designed to do nothing but carry dry bulk. A geared vessel, with cranes and the structure to support them, or a vessel with unusually large machinery or ballast spaces, will show a lower NT-to-GT ratio. The DWT bands are repeated here for orientation only; deadweight is the weight measure that defines the class commercially, while GT and NT are the volume indices that drive dues and regulation. Every GT, NT and ratio figure in this table is representative and is flagged for desk verification.

Common confusions about tonnage

Tonnage attracts a cluster of recurring mistakes. The most common, in roughly the order the desk sees them:

• Tonnage is volume, not weight. GT and NT are derived from enclosed cubic metres, then converted into a dimensionless index. They are not measured in tonnes and they do not tell you how heavy the ship is. The weight measures are displacement (the ship’s own weight) and deadweight (what she can carry).
• GT is an index, not a count of tonnes. A “30,000 GT” vessel does not contain 30,000 of anything. The number is the output of the K1 × V formula, with no unit. Treating it as a weight or a cargo figure produces nonsense in a voyage estimate.
• GT and NT versus DWT. Charterers fixing cargo care about deadweight, because that is what limits the parcel. GT and NT matter to the operator paying dues and to the regulator setting manning, not to the size of the stem. Quoting GT when the counterparty wants DWT is a frequent cross-talk error.
• “Tonnage” in market chatter. Brokers and the trade press use “tonnage” loosely to mean “ships” or “available vessel supply”, as in “Atlantic tonnage is tight”. That colloquial usage has nothing to do with the measured GT or NT figure on a certificate. Context tells you which sense is meant.
• Canal tonnage versus ITC 69 tonnage. Suez Canal Net Tonnage and PC/UMS net tonnage are separate, certificate-specific figures used only for canal tolls. They differ from the ITC 69 NT and from each other, and they should never be substituted for the certified ITC figure.

Scope and what this page does not cover

This page explains gross and net tonnage as volumetric ship measures under ITC 69, how they differ from the weight measures, and how they feed dues, regulation and canal tolls. It does not reproduce the full ITC 69 measurement procedure, calculate a specific vessel’s GT or NT, quote current Suez or Panama toll rates, or advise on tonnage-tax treatment in any jurisdiction. For the weight measures see deadweight tonnage and the per-class specifications on the bulk-carriers hub. For a specific vessel’s certified figures, work from her International Tonnage Certificate and the classification society record.