Gross tonnage

 ( Shipbuilding ) 

Various methods were previously used to calculate merchant ship tonnage, but they differed significantly and one single international system was needed. Previous methods traced back to Moorsom System of Great Britain's Board of Trade who devised one such method in 1854.

The tonnage determination rules apply to all ships built on or after 18 July 1982. Ships built before that date were given 12 years to migrate from their existing gross register tonnage (GRT) to use of GT and NT. 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. Tonnage is also the basis for calculating registration fees and port dues. One of the convention's goals was to ensure that the new calculated tonnages "did not differ too greatly" from the traditional gross and net register tonnages.

Both GT and NT are obtained by measuring ship's volume and then applying a mathematical formula. Gross tonnage is based on "the moulded volume of all enclosed spaces of the ship" whereas net tonnage is based on "the moulded volume of all cargo spaces of the ship". In addition, a ship's net tonnage is constrained to be no less than 30% of her gross tonnage.

• V, the ship's total volume in cubic metres (m³), and
• K, a multiplier based on the ship volume.

The value of the multiplier K increases logarithmically with the ship's total volume (in cubic metres) and is applied as an amplification factor in determining the gross tonnage value. K is calculated with a formula which uses the common logarithm:

$K\; =\; 0.2\; +\; 0.02\; \backslash times\; \backslash log\_\{10\}(V)\backslash ,$

Once V and K are known, gross tonnage is calculated using the formula, whereby GT is a function of V:

$GT\; =\; V\; \backslash times\; K\backslash ,$

which by substitution is:

$GT\; =\; V\backslash times(0.2\; +\; 0.02\backslash times\backslash log\_\{10\}(V))$

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.

$V\; =\; \backslash frac\{50\; \backslash times\; \backslash ln\; 10\; \backslash times\; GT\}\{W(5\; \backslash times\; 10^\{11\}\; \backslash times\; \backslash ln\; 10\; \backslash times\; GT)\}$

• Compensated gross tonnage
• List of world's largest ships by gross tonnage
• Ton
• Tonnage

• William B. Hayler (2025). 9780870335495, Cornell Maritime Press. ISBN 9780870335495
• (1980). 9780870330568, Cornell Maritime Press. ISBN 9780870330568