By deducting the volume of non-earning spaces (e. g. crew accommodation) from the gross registered tonnage (grt) measurement. (Under the “old” Moorsom measurement system, 100 cu ft = 1 gross register ton; this is still the basis of the method used for determining Suez Canal tonnage.)
How was net tonnage determined in the UK before ratification of the International Tonnage Convention, 1969?
