Long ton, also known as the imperial ton or displacement ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements standardised in the thirteenth century that is used in the United Kingdom and several other British Commonwealth of Nations countries alongside the mass-based metric ton invented in 1799.
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Unit definition
A long ton is defined as exactly 2,240 pounds. The long ton arises from the traditional British measurement system: A long ton is 20 cwt, each of which is 8 stone (1 stone = 14 pounds). Thus a long ton is 20 × 8 × 14 lb = 2,240 lb.
Unit equivalences
A long ton, also called the weight ton (W/T) is equal to 2,240 pounds (1,016 kg), coincidentally equivalent to the mass of 35 cubic feet (0.99 m3) of salt water with a density of 64 pounds per cubic foot (1.03 g/cm3). One long ton is 1.12 short tons or 12% larger than the 2,000 pounds of the North American short ton, and 1.6% larger than the 1,000-kilogram (2,205 lb) tonne (metric ton).
It has some limited use in the United States, most commonly in measuring the displacement of ships, the volume-to-carrying-weight of fuels and in trade of baled commodities and bulk goods like elemental sulfur. The long ton was the unit prescribed for warships by the Washington Naval Treaty 1922—for example battleships were limited to a displacement of 35,000 long tons (36,000 t; 39,000 short tons).
To comply with the practices of the European Union, the British Imperial ton was explicitly excluded from use for trade by the United Kingdom's Weights and Measures Act of 1985.
International usage
In order to avoid confusion, especially in international environments, it is recommended to always use the full name: "short ton", "long ton" or "metric tonne".