St. Petersburg, Florida

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Small Boat/Vessel Gross Tonnage Explained

The term "Gross Tons" is used by the Coast Guard in licensing and vessel documentation and inspection procedures. Gross tonnage, now called Gross Register Tons (GRT) or Domestic Tons, is a term used to describe the total enclosed space (internal capacity) of a vessel, using 100 cubic feet per ton. Net tonnage is a gross tonnage less fuel compartments, engine space, wheelhouse, crew’s quarters and other spaces having cargo carrying capacity.

Gross tonnage is calculated by multiplying overall length (L), by overall breadth (B), by depth (D) (not draft but internal depth from keel to inside cabin top).

Divide the product of L x B x D ÷ 100. For a sailing vessel use ½ of the answer, and for a power (motor) vessel use 2/3 of the answer. See example formula:

Gross Tonnage (Sail)    =    (L x B x D ÷ 100) x .50

Gross Tonnage (Power)    =    (L x B x D ÷ 100) x .67

Net Tonnage (Sail)    =    Gross Tonnage x .90

Net Tonnage (Power)    =    Gross Tonnage x .80

For sail vessels if there is no propelling machinery in the hull then Gross Tonnage = Net Tonnage.

These calculations are not accurate for vessels over 40 feet in length. Vessel documentation papers are the only reliable source of information on gross and net tonnage. Undocumented vessels under 40 feet in length may be considered as being under 5 net tons.

FOR MORE DETAILS ON TONNAGE EXPLAINED CLICK ON THE BELOW LINK TO VIEW, DOWNLOAD OR PRINT THE U.S. COAST GUARD'S PUBLICATION -"TONNAGE GUIDE FOR SIMPLIFIED MEASUREMENT."