46 CFR § 69.209 - Gross and net register tonnages.
(a) Gross register tonnage.
(1) Except as in paragraphs (a)(2) through (a)(5) of this section, the gross register tonnage of a vessel designed for sailing is one-half of the product of its overall length, overall breadth, and overall depth (LBD) divided by one hundred (i.e., 0.50 LBD/100), and the gross register tonnage of a vessel not designed for sailing is 0.67 LBD/100.
(2) The gross tonnage of a vessel with a hull that approximates in shape a rectangular geometric solid (barge-shape) is 0.84 LBD/100.
(3) The gross tonnage of a multi-hull vessel is the sum of all the hulls as calculated under this section.
(4) If the volume of the principal deck structure of a vessel is as large as, or larger than, the volume of the vessel's hull, the volume of the principal deck structure in tons of 100 cubic feet is added to the tonnage of the hull to establish the vessel's gross tonnage. The volume of the principal deck structure of a vessel is determined by the product of its average dimensions.
(5) If the overall depth of a vessel designed for sailing includes the keel, only 75 percent of that depth is used for gross tonnage calculations.
(b) Net register tonnage.
(1) For a vessel having propelling machinery in its hull—
(i) The net register tonnage is 90 percent of its gross register tonnage, if it is a vessel designed for sailing; or
(ii) The net register tonnage is 80 percent of its gross register tonnage, if it is not a vessel designed for sailing.
(2) For a vessel having no propelling machinery in its hull, the net register tonnage is the same as its gross register tonnage.
(c) Certification of measurement. For a vessel that is documented as a vessel of the United States under part 67 of this chapter, the vessel's Certificate of Documentation serves as evidence of measurement under this subpart. For all other vessels, a completed Application for Simplified Measurement (form CG–5397) serves as evidence of the tonnage measurement under this part.