Usually, If It’s At Least 25 Feet Long, It Measures Five Net Tons Or More  Net tonnage determines a vessel's volume, distinct from its weight, which may also be expressed in tons. Access to this information should be available via the manufacturer’s data or a gross tonnage calculator, but an approximation can also be calculated independently. A useful guideline is that vessels longer than 25 feet typically have a net tonnage of at least five tons or more. Mandatory for registration, vessel tonnage must be measured as dictated by the Vessel Registration and Tonnage Regulations and TP 13430 (Standard for the Tonnage Measurement of Vessels) by an officially appointed Tonnage Measurer.