46 CFR §69.107

1. (a)The vessel's gross register tonnage is the sum of the following tonnages, less the tonnages of certain spaces exempt under § 69.117:
   1. (1)Under-deck tonnage (§ 69.109).
   2. (2)Between-deck tonnage (§ 69.111).
   3. (3)Superstructure tonnage (§ 69.113).
   4. (4)Excess hatchway tonnage (§ 69.115(c)).
   5. (5)Tonnage of framed-in propelling machinery spaces included in calculating gross tonnage (§ 69.121(d)(1)).
2. (b)The vessel's net register tonnage is the 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.