Net tonnage

The net tonnage calculation is based on a number of factors, one of which is the moulded draft d. The choice of the value to use for d can be complicated. For ships subject to the International Convention on Load Lines, the Summer Load Line draft is used, with the exception of cases where that is a timber load line. For passenger ships, the draft used is the deepest subdivision load line assigned in accordance with the International Convention for the Safety of Life at Sea. Otherwise, if a ship has been assigned a load line by its national government, the draft for that summer load line is used. If the ship has no load line, instead, a maximum draft assigned by its national government, that value is used, if it has been assigned a maximum. Finally, for a ship to which none of the above applies, the value of d is taken as 75 per cent of the moulded depth amidships.

The Net tonnage calculation is defined in Regulation 4 of Annex 1 of The International Convention on Tonnage Measurement of Ships, 1969. It is based on three main variables:

• V_c, the total volume of the ship's cargo spaces in cubic meters (m³),
• d, the ship's moulded draft amidships in meters, and
• D, the ship's moulded depth amidships in metres

The first step in calculating NT is to find the value known as K₂, a multiplier based on V_c. It is obtained by using the following formula:

${\displaystyle K_{2}=0.2+0.02\times \log _{10}(V_{c})}$

And then these three values are used to calculate NT using this formula:

${\displaystyle NT=K_{2}\times V_{c}\times ({\tfrac {4d}{3D}})^{2}}$

Where the factor ${\displaystyle ({\tfrac {4d}{3D}})^{2}}$ will not exceed 1, the term ${\displaystyle K_{2}\times V_{c}\times ({\tfrac {4d}{3D}})^{2}}$ will not be less than 0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.

When calculating NT for ships certified to carry 13 or more passengers, an additional term is used in the NT formula. It is based on three additional variables:

• GT, the ship's gross tonnage,
• N₁, number of passengers in cabins with not more than 8 berths, and
• N₂, number of other passengers,

First, a multiplier K₃, based on the ship's gross tonnage is found,

${\displaystyle K_{3}={\frac {1.25\times (GT+10000)}{10000}}}$

Then the net tonnage is calculated:

${\displaystyle NT=K_{2}\times V_{c}\times ({\tfrac {4d}{3D}})^{2}+K_{3}\times (N_{1}+{\frac {N_{2}}{10}})}$

Where the factor ${\displaystyle ({\tfrac {4d}{3D}})^{2}}$ shall not be taken as greater than unity, the term ${\displaystyle K_{2}\times V_{c}\times ({\tfrac {4d}{3D}})^{2}}$ will not be less than 0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.

The difference between the cases of 12 of fewer passengers and 13 or more passengers is due to a restriction given in the net tonnage definition that states "...when N₁ + N₂ is less than 13, N₁ and N₂ shall be taken as zero."