How do you find the gross tonnage of a vessel?

The gross tonnage calculation is defined in Regulation 3 of Annex 1 of The International Convention on Tonnage Measurement of Ships, 1969. It is based on two variables, and ultimately a one-to-one function of ship volume:

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

The value of the multiplier K varies in accordance with a ship's total volume (in cubic meters) and is applied as a kind of reduction factor in determining the gross tonnage value - which does not have a unit such as cubic meters or tons. For smaller ships, K is smaller, for larger ships, K is larger. K ranges from 0.22 to 0.32 and is calculated with a formula which uses the common or base-10 logarithm:

K = 0.2 + 0.02 x log 10 (V)

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

GT = K x V

As an example, we can calculate the gross tonnage of a ship with 10,000 m^3 total volume.

K = 0.2 + 0.02 x log 10 (10,000) = 0.2 + 0.02 x 4 = 0.2 + 0.08 = 0.28

Then the gross tonnage is calculated:

0.28 x 10,000 = 2800 GT