Gross tonnage is measure of the total enclosed volume of the ship in cubic metres multiplied by a constant, and the net tonnage (or Register tonnage) is the total enclosed volume available for cargo in cubic metres multiplied by a constant.
Port dues, canal dues and various government dues are often levied on these measurement tonnages. Shipowners, naturally anxious to keep the dues they paid down to a minimum, instructed their naval architects to study the rules carefully so as to build a ship of a certain capacity with the smallest gross tonnage possible. In order to encourage safety, governments would offer various exempted spaces as an incentive for good building practices: for instance, the double bottom was exempted from gross tonnage if it was used only for water ballast. The precise definitions of measurement tonnage had therefore become long, detailed and fairly complex. To add to this seeming confusion, different countries had developed different rules for making these measurements. Prior to 1982 there were about five basic systems in use: British, American, most other maritime nations, the Suez Canal Authority and the Panama Canal Authority. However, ships built before 1982 were allowed to retain their original GRT and NRT, if the owners so wished, until 18 July 1994.
Gross registered tonnage was the total internal capacity below the uppermost continuous deck, the ‘tonnage’ deck for most ship, plus all permanent enclosed spaces above this deck, less exempted spaces. Examples of exempted spaces were double bottom tanks, if used only for water ballast, and crew ablution rooms above the upper deck. Net registered tonnage was the gross tonnage less deducted spaces. Examples of deducted spaces included machinery spaces, crew accommodation, chart room and radio room.