- Gross tonnage – Under vessel measurement rules of various nations, the Panama Canal and the Suez Canal gross tonnage is a measure of the internal volume of space within a vessel in which 1 ton is equivalent to 2.83 cub m or 100 cub ft. Under the International Convention on Tonnage Measurement of Ships, 1969, (ICTM), a standardized numerical value is a logarithmic function of spaces within a vessel. There is no definition of a ton under ICTM because the value per unit of volume is greater on a vessel of large volume than on a vessel of small volume. Gross tonnage according to the national and canal rules generally includes spaces bounded by the under surface of the uppermost complete deck, the side frames, and the floor frames or the inner bottom if it rests on the floors or if the double water is for water ballast, plus closed-in space in deck structures are available for cargo or stores or for the berthing or accommodation of passengers or crew. Rules vary greatly as to exclusion or inclusion of various spaces. Gross tonnage according to ICTM is GT = K1 V in which V is the total moulded volume of all enclosed spaces of the ship in cub m and K1 is 0.2 + 0.02 log10 V.
The use of gross tonnage as a criterion for charging port and other dues, and the efforts of designers make it minimal, leads to the whole range of undesirable features in ship design.
- Net tonnage – Net tonnage according to the national and canal rules is derived from gross tonnage by deducting an allowance for the propelling machinery space and certain other spaces. Net tonnage according to ICTM is a logarithmic function of the volume of cargo space, the draft-to-depth ratio, the number of passengers to be accommodated, and the gross tonnage.