The tonnage of ships above 24 metres in length *) which are engaged on international
voyages is determined according to the International Convention on Tonnage Measurement
of Ships, 1969 (London-Rules). Under the Convention system, the volume of all
ship’s enclosed spaces (from keel to funnel) is measured to the inside of the
hull framing, whereas special open spaces are excluded, so they are not considered
in the total volume.
measurement according to the London 69 Rules
The dimensionless gross tonnage (GT) is computed from the total volume V (in
m3) multiplied by the factor K1 (from 0.22 up to 0.32), using the formula GT
= K1 * V. The gross tonnage indicates a vessel’s total volume, not her weight
or deadweight, which is a common misconception.
The net tonnage (NT) indicates a vessel’s earning space. NT is also dimensionless
and depends on the total volume of all cargo spaces, the draught, moulded depth
and number of passengers. Net tonnage is determined using a special formula
and is not allowed to be less than 0.3 GT.
According to IMO-regulations a reduced gross tonnage is certified for 'open
top' containerships and tanker with a double hull (ships equipped with segregated
ballast tanks SBT).
*) The definition of length, as given in the International Convention on
Load Lines, 1966, means ”96 per cent of the total length on a waterline at 85
per cent of the least moulded depth measured from the top of the keel, or the
length from the fore side of the stem to the axis of the rudder stock on that
waterline, if that be greater”.