Tonnage decides how much a ship pays in port, how many crew it needs, and what it costs to transit a canal. Yet two of the most important figures, gross tonnage and net tonnage, are widely misunderstood, often confused with each other and with the ship's weight. 

Gross Tonnage vs Net Tonnage: The Short Answer

Gross tonnage measures the total enclosed volume of an entire ship, while net tonnage measures only the volume of its cargo-carrying spaces. Both are measures of volume, not weight.

Put simply, gross tonnage reflects the overall size of the vessel, and net tonnage reflects its earning capacity. A ship's net tonnage is always lower than its gross tonnage, typically at least 30 percent of the gross figure.

Both measurements were standardized by the International Convention on Tonnage Measurement of Ships, 1969, adopted by the IMO and in force since July 18, 1982. The convention replaced the older gross register tonnage (GRT) and net register tonnage (NRT) with a single universal system.

What Is Gross Tonnage?

Gross tonnage (GT) is a measure of a ship's total internal volume, calculated from the molded volume of all enclosed spaces on the vessel. The figure covers everything inside the hull and superstructure: cargo holds, engine rooms, crew accommodation, storerooms, and bridge.

Gross tonnage is a nonlinear figure, derived using a formula based on the ship's total enclosed volume rather than a simple measurement. Because it captures the whole vessel, gross tonnage is the standard reference for a ship's overall size. A larger gross tonnage means a physically larger ship.

What Is Net Tonnage?

Net tonnage (NT) is a measure of the volume of a ship's cargo-carrying spaces, the part of the vessel that actually earns revenue. Net tonnage starts from the cargo volume and excludes non-earning spaces such as the engine room, fuel tanks, and crew quarters.

Because it isolates usable cargo space, net tonnage reflects the commercial or earning capacity of the ship. The 1969 Convention sets a floor on the figure, so net tonnage cannot fall below 30 percent of gross tonnage. Ships with high freeboard and low draft-to-depth ratios would otherwise produce unrealistically low net tonnages.

Gross Tonnage vs Net Tonnage: Side by Side

The clearest way to separate the two is to compare what they measure and how they are used. The key point is that gross tonnage describes the ship, while net tonnage describes its cargo capacity.

Neither figure is a weight. Both express volume, which is why a heavily built vessel and a light one of the same internal size can share the same tonnage.

How Gross and Net Tonnage Are Used

Each figure drives different decisions in shipping, which is why both appear on a ship's tonnage certificate. The distinction matters most when fees and regulations are assessed.

Gross tonnage is commonly used for:

• Manning regulations and crew requirements.
• Safety rules and the conventions that apply to a vessel.
• Ship registration fees.
• Many port dues and charges.

Net tonnage is commonly used for:

• Canal transit tolls, such as the Suez and Panama Canals.
• Harbor and port dues based on earning capacity.
• Charter and freight calculations that reference cargo space.

Because tonnage determines which safety and manning rules apply, it connects directly to a vessel's SOLAS and IMO compliance obligations. Operators rely on accurate tonnage figures to budget voyages and stay compliant.

Tonnage Is Not Weight

A common error is treating tonnage as a measure of weight. The key point is that gross and net tonnage are volume measurements, while weight-based figures are entirely different.

Two other terms describe mass or weight: deadweight tonnage, the total weight a ship can safely carry including cargo, fuel, and stores, and displacement, the weight of the water the ship displaces. Confusing these with gross or net tonnage leads to mistakes in everything from loading to fee calculation. Accurate operations depend on knowing which figure a regulation or contract actually refers to, and pairing that with current nautical charts and publications for safe passage planning.

Know Your Numbers, Run a Tighter Ship

Gross tonnage and net tonnage answer two different questions: how big is the ship, and how much can it earn. Getting them right keeps port dues accurate, manning compliant, and canal tolls predictable. For IMO tonnage and compliance publications, charts, and maritime reference materials, contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

Q. What is the difference between gross tonnage and net tonnage?

Gross tonnage measures the total enclosed volume of an entire ship, including cargo holds, engine room, and accommodation. Net tonnage measures only the volume of cargo-carrying spaces, reflecting the ship's earning capacity. Both are volume measures, not weight.

Q. Is tonnage a measure of weight?

No. Gross tonnage and net tonnage are measures of volume. Weight-based measures are separate: deadweight tonnage is the weight a ship can carry, and displacement is the weight of water the ship displaces.

Q. How are gross and net tonnage calculated?

Both are defined by the International Convention on Tonnage Measurement of Ships, 1969. Gross tonnage is derived from the molded volume of all enclosed spaces using a formula, and net tonnage is derived from cargo space volume and cannot fall below 30 percent of gross tonnage.

Q. Why is net tonnage lower than gross tonnage?

Net tonnage counts only cargo-carrying spaces and excludes the engine room, fuel tanks, and crew quarters, so it is always lower than gross tonnage. The 1969 Convention sets a minimum of 30 percent of the gross tonnage figure.

Q. What is gross tonnage used for?

Gross tonnage is used to determine manning requirements, safety regulations, ship registration fees, and many port dues. The figure serves as the standard reference for a vessel's overall size.

Q. What is net tonnage used for?

Net tonnage is used for canal transit tolls, such as the Suez and Panama Canals, harbor and port dues based on earning capacity, and charter and freight calculations that reference cargo space.