How is the net tonnage of a cargo ship determined under the International Tonnage Convention, 1969?

By measurement of the volume, in cubic metres, of all the cargo spaces in the ship. 

This volume is multiplied by a “multiplier” (K) obtained from a table in the Convention (or in The Merchant Shipping (Tonnage) Regulations 1982) and then by the square of 4d/3D, where d is the ship’s moulded summer draught and D is the moulded depth. The net tonnage must be at least 30 per cent of the gross tonnage and it may not be changed more than once a year.