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Gross tonnage is a measurement of the internal volume of a ship for the purpose of determining which regulations apply, calculating port fees, and so forth. Although this term sounds like a reference to weight, because it uses the term “ton,” it is important to be aware that it is a volume measurement, and does not provide information about the weight of a ship, laden or empty. This is one among a group of terms used to describe the characteristics of ships for regulatory purposes.
Each “ton” in this unit of measurement consists of 100 cubic feet (2.83 cubic meters) of space. There are several ways to calculate gross tonnage, looking at ways different cargoes might fit into a ship. Bales and shipping containers will need to stack and take up space differently than something like a hold filled with grain, where the grain can fill every corner of the hold. In gross tonnage disclosures, a discussion of the calculation approach used informs readers so they can understand the data they are examining.
The gross tonnage does not include structures above the decks and also excludes spaces set aside for crew, passengers, ballast, and gear. It focuses specifically on the cargo holds. The higher the gross tonnage, the more the ship can carry. The ship will also have a separate weight rating indicating the maximum weight it can safely carry, as well as the unladen weight. Other useful measures include things like displacement.
This measurement can affect the ports a ship may enter, how much it pays to dock, and what kinds of registration fees are due. These fees are scaled by size to ensure that ships pay a reasonable share on the basis of how much they carry. Manning requirements and other legal stipulations are also based on gross tonnage. Ships must disclose their measurements so authorities can make sure the owners and captains are handling their vessels responsibly and complying with any relevant laws.
Measurements of gross tonnage can be determined during the design and engineering phase. Designers want to make sure a ship will meet a stated need and can vary the design to adjust the tonnage. Customers may have particular requests depending on what they carry and where they travel. Knowing the ship's basic characteristics ahead of time may be important for securing loans, insurance, and other necessary support before taking delivery of a new ship.