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HMRC internal manual

Tonnage Tax Manual

HM Revenue & Customs
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Background material: Ships


Gross tonnage

‘Gross (registered) tonnage’ (GT or GRT) forms the basis for manning regulations, safety rules, registration fees and is also used to calculate port dues and fees for the transit of canals.  Gross tonnage is the measurement of total volume of all enclosed spaces in a ship, with one gross ton equalling 100 cubic feet.

Gross tonnage of 100 tons or over is a requirement for a ship to be qualifying.

Net tonnage

‘Net (or nett) (registered) tonnage’ (NT or NRT) represents the earning capacity of the vessel, as it is based on the amount of space available for cargo or passengers, excluding, for example, space occupied by the engines or the bunkers.  It is based on a similar measurement to gross tonnage, i.e. 1 net ton equals 100 cubic feet.

Net tonnage is used in the calculation of the tonnage tax profit.

Other tonnage measurements

‘Deadweight Tonnage’ (DWT) is often used as a measurement of cargo-carrying capacity.  Unlike GT and NT, which measure volume, it measures weight.  The weight is of all cargo, stores, bunkers, fresh water, food, crew and passengers, which would bring a ship down to her summer load line.  It is generally calculated by reference to 1,000 kg as one metric tonne.

‘Lightweight Tonnage’ (LWT) (or light displacement tonnage) is used to measure the scrap metal content of a ship destined to be broken up.  Again, it is a measure of weight, this time of the ship’s hull and machinery.

Neither measurement has any relevance to tonnage tax.


Qualifying ship of 100 gross tons or more TTM03500
Calculation of tonnage tax profit using net tonnage TTM01300
Measurement of tonnage for tonnage tax TTM01330
Certification of ships TTM15030