Gross tonnage is a measure of the internal capacity of the ship and net tonnage is intended to give an idea of the earning or useful capacity of the ship.
Various port dues and other charges may be assessed on the gross and net tonnages.
The gross tonnage (GT) is determined by the following formula:
GT = K1 V
K1=0.2+0.02 log10 V
V=total volume of all enclosed spaces in cubic metres.
The net tonnage (NT) is determined by the following formula:
- For passenger ships (i.e. ships carrying 13 passengers or more)
- For other ships:
Vc = Total volume of cargo spaces in cubic metres.
d = Moulded draft amidships in metres (summer load line draft or deepest subdivision load line in case of passenger ships).
D = Moulded depth in metres amidships.
N1 = Number of passengers in cabins with not more than 8 berths.
N2=Number of other passengers.
N1+N 2 = Total number of passengers the ship is permitted to carry. The factor is not taken to be greater than unity.
The term K2 Vc is not to be taken as less than 0.25 GT; and NT is not to be taken as less than 0.30 GT.
It will be noted that vessels with high freeboards, i.e. low draft to depth (d/D) ratios will have low net tonnages. Squaring this ratio can result in excessively low net tonnages hence the limiting value of 0.30 GT.