46 CFR 69.161: Gross and net register tonnages.

(a) Gross register tonnage means the tonnage of a vessel, less certain spaces exempt under § 69.169, and is the sum of the following:

(1) Under-deck tonnage (§ 69.163).

(2) Between-deck tonnage (§ 69.165)

(3) Superstructure tonnage (§ 69.167)

(4) Excess hatchway tonnage (§ 69.115)

(5) Tonnage of framed-in propelling machinery spaces included in calculating gross register tonnage (§ 69.121(d)(1)).

(b) Net register tonnage means gross register tonnage less deductions under § 69.119 and § 69.121.

(c) The authorized measurement organization must issue a U.S. Tonnage Certificate as evidence of a vessel's measurement under this subpart, which must also indicate the vessel's measurement under the Convention Measurement System in subpart B of this part, if applicable. There is no requirement to maintain the U.S. Tonnage Certificate on board the vessel.