46 CFR §69.161

1. (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. (1)Under-deck tonnage (§ 69.163).
   2. (2)Between-deck tonnage (§ 69.165)
   3. (3)Superstructure tonnage (§ 69.167)
   4. (4)Excess hatchway tonnage (§ 69.115)
   5. (5)Tonnage of framed-in propelling machinery spaces included in calculating gross register tonnage (§ 69.121(d)(1)).
2. (b)Net register tonnage means gross register tonnage less deductions under § 69.119 and § 69.121.
3. (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.