Gross tonnage

Gross tonnage is calculated based on "the moulded volume of all enclosed spaces of the ship" and is used to determine things such as a ship's manning regulations, safety rules, registration fees, and port dues, whereas the older gross register tonnage is a measure of the volume of only certain enclosed spaces.

[2] Various methods were previously used to calculate merchant ship tonnage, but they differed significantly and one single international system was needed.

[2] Ships built before that date were given 12 years to migrate from their existing gross register tonnage (GRT) to use of GT and NT.

[2] The phase-in period was provided to allow ships time to adjust economically, since tonnage is the basis for satisfying manning regulations and safety rules.

K is calculated with a formula which uses the common or base-10 logarithm: Once V and K are known, gross tonnage is calculated using the formula, whereby GT is a function of V: which by substitution is: Thus, gross tonnage exhibits linearithmic growth with volume, increasing faster at larger volumes.

The units of gross tonnage, which involve both cubic metres and log-metres, have no physical significance, but were rather chosen for historical convenience.