Gross tonnage (GT, G.T. or gt) is a nonlinear measure of a ship's overall internal volume. Gross tonnage is different from gross register tonnage.^{[1]} Neither gross tonnage nor gross register tonnage should be confused with measures of mass or weight such as deadweight tonnage or displacement.
Gross tonnage, along with net tonnage, was defined by the International Convention on Tonnage Measurement of Ships, 1969, adopted by the International Maritime Organization (IMO) in 1969, and came into force on 18 July 1982. These two measurements replaced gross register tonnage (GRT) and net register tonnage (NRT). Gross tonnage is calculated based on "the moulded volume of all enclosed spaces of the ship" and is used to determine things such as a ship's manning regulations, safety rules, registration fees, and port dues, whereas the older gross register tonnage is a measure of the volume of only certain enclosed spaces.
YouTube Encyclopedic

1/3Views:18 19333 0741 414

TONNAGE and TONNES Explained. ( GT, GRT, NT, NRT, LIGHT WEIGHT and DEAD WEIGHT )

Ship's Tonnage Explained.

GRT NRT Tonnage measurements
Transcription
History
The International Convention on Tonnage Measurement of Ships, 1969 was adopted by IMO in 1969.^{[2]} The Convention mandated a transition from the former measurements of gross register tonnage (grt) and net register tonnage (nrt) to gross tonnage (GT) and net tonnage (NT).^{[2]} It was the first successful attempt to introduce a universal tonnage measurement system.^{[2]}
Various methods were previously used to calculate merchant ship tonnage, but they differed significantly and one single international system was needed.^{[2]} Previous methods traced back to George Moorsom of Great Britain's Board of Trade who devised one such method in 1854.^{[2]}
The tonnage determination rules apply to all ships built on or after 18 July 1982.^{[2]} Ships built before that date were given 12 years to migrate from their existing gross register tonnage (GRT) to use of GT and NT.^{[2]} The phasein period was provided to allow ships time to adjust economically, since tonnage is the basis for satisfying manning regulations and safety rules.^{[2]} Tonnage is also the basis for calculating registration fees and port dues.^{[2]} One of the convention's goals was to ensure that the new calculated tonnages "did not differ too greatly" from the traditional gross and net register tonnages.^{[2]}
Both GT and NT are obtained by measuring ship's volume and then applying a mathematical formula.^{[2]} Gross tonnage is based on "the moulded volume of all enclosed spaces of the ship" whereas net tonnage is based on "the moulded volume of all cargo spaces of the ship".^{[2]} In addition, a ship's net tonnage is constrained to be no less than 30% of her gross tonnage.^{[2]}
Calculation
The gross tonnage calculation is defined in Regulation 3 of Annex 1 of The International Convention on Tonnage Measurement of Ships, 1969.^{[3]} It is based on two variables, and is ultimately an increasing onetoone function of ship volume:
 V, the ship's total volume in cubic metres (m^{3}), and
 K, a multiplier based on the ship volume.
The value of the multiplier K varies in accordance with a ship's total volume (in cubic metres) and is applied as an amplification factor in determining the gross tonnage value. For smaller ships, K is smaller, for larger ships, K is larger. K is calculated with a formula which uses the common or base10 logarithm:
Once V and K are known, gross tonnage is calculated using the formula, whereby GT is a function of V:
which by substitution is:
Note that the units of gross tonnage, which involve both cubic metres and logmetres, have no physical significance, but were rather chosen for historical convenience.
The inverse, on the other hand, is not as simple. A rootfinding algorithm may be used for obtaining an approximation to a ship's volume given its gross tonnage. The exact formula is:
where is the natural logarithm and is the Lambert W function.
Gross tonnage  Volume (m^{3})  Ratio (1/K) 

0.2  1  5 
2.2  10  4.545 
24  100  4.167 
260  1,000  3.846 
2800  10,000  3.571 
30000  100,000  3.333 
320000  1,000,000  3.125 
See also
Notes
 ^ The gross register ton (GRT) is a unit of volume defined as 100 cubic feet (2.8 m^{3}).
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} ^{g} ^{h} ^{i} ^{j} ^{k} ^{l} ^{m} International Maritime Organization, 1982.
 ^ "International Convention on Tonnage Measurement of Ships, 1969". Admiralty and Maritime Law Guide. London. 23 June 1969.
References
 "International Convention on Tonnage Measurement of Ships, 1969". 19690623. Retrieved 20080323.
 International Maritime Organization (1982). "International Convention on Tonnage Measurement of Ships, 1969". International Maritime Organization. Retrieved 20180819.
 Hayler, William B. (2003). American Merchant Seaman's Manual. Cornell Maritime Press. ISBN 0870335499.
 Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 9780870330568.