Calculating the size of cargo in a ship according to the volume is referred to as tonnage calculation. Tonnage refers to the unit of ship's volume while Ton is the unit of weight.

Tonnage Calculations governed by IMO Convention

Tonnage measurements range from Gross Register Tonnage (GRT), Net Register Tonnage (NRT), Gross Tonnage, Net Tonnage to Thames Measurement Tonnage. Gross Register Tonnage is the total volume of vessel excluding the unused spaces while Net Register Tonnage is the total cargo a vessel can carry.

Gross Tonnage is explained as the volume of ship's entire enclosed spaces till the exterior of hull whereas Net Tonnage calculates all the cargo spaces of the ship. Last of all, the Thames measurement tonnage used for smaller vessels is a formula based on calculating the vessel's length and beam. Gross Register Tonnage and Net Register Tonnage were replaced by Gross and Net Tonnage respectively in 1994 according to the Tonnage Measurement convention of 1969 which succeeded to bring out a universal tonnage measurement system. Since 1982 all ships are characteristically measured by IMO Convention.

According to IMO Convention, GT or Gross Tonnage is calculated by the formula:

GT = K.V
K= a figure from 0.22 to 0.32 which is determined over ship's range
V= Volume

One Gross Register Ton is a volume of 100 cubic feet or say 2.83 m³. GT is always smaller than the value of GRT as:

0.5919 GT = 1 GRT which is 2.83 m³.