Units of mass
There are three similar units of mass called the ton:
long ton (simply ton in countries such as the United Kingdom which formerly used the Imperial system of weights and measures) is a weight ton or gross ton, and is 2,240 lb (exactly 1,016.0469088 kg). In the UK and most of the areas which used the Imperial system, the metric tonne (1,000 kg), which it is conveniently very similar to—less than 2% difference—is the only form of ton legal for trade.
In the iron industry in the 17th century and 18th century, a ton shortweight was the standard 2,240 lb, whereas a ton longweight was 2,400 lb (the hundredweight being 120 lb).
The long ton is used for petroleum products such as aviation fuel.
Deadweight ton (abbreviation 'dwt') is a measure of a ship's carrying capacity, including bunker oil, fresh water, crew and provisions. It is expressed in metric tons (1,000 kg) or long tons (2,240 pounds, about 1,016 kg). This measurement is also used in the U.S. tonna