This depends on the measurement system used. While in all cases, the method for calculating tonnage is detailed in the tonnage regulations, Formal System rules are complex, and measurement is best left to individuals experienced in using these rules (e.g., authorized measurement organizations). Conversely, Simplified System rules are more straightforward, and the formulas for calculating tonnage have been programmed into the electronic Adobe .pdf version of the Simplified Application form. An owner can ascertain net tonnage by entering vessel data into this form, with the tonnage appearing in the form’s upper left hand corner.
How do I choose a measurement system?
This depends on the circumstances specific to your vessel. In general, owners of most vessels less than 79 feet in overall length opt for the Simplified System, as this system often yields tonnages meeting their objectives while avoiding the cost of Formal measurement. Please be aware that under U.S. law, if a vessel is not formally measured, the Simplified System applies by default, regardless of whether an Application for Simplified Measurement (form CG-5397) is completed.
Where can I find evidence of prior measurement?
For documented vessels, the assigned tonnages appear on the COD, regardless of the measurement system used, and can also be found in the Coast Guard’s publically available PSIX vessel information system. For other vessels, there is no corresponding federal document or data system that provides such evidence of tonnage.
The Coast Guard recommends that a copy of the tonnage certificate (if formally measured) or a completed Simplified Application (if simplified measured) be carried onboard any undocumented commercial vessel less than 79 feet in overall length for presentation to law enforcement personnel.
For further information, refer to the U.S. Documentation and Tonnage Regulations (46 CFR 67 and 46 CFR 69, respectively), and the websites of the following governmental organizations: NVDC, MSC, CBP, and MARAD. In addition, the MSC maintains useful links to related documents and web pages on its website: http://www.uscg.mil/hq/msc/tonnage/links.asp.