Long ton, also known as the imperial ton or weight 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 Commonwealth countries alongside the French metrication invented in 1799.he long ton arises from the traditional British measurement system: A long ton is 20 hundredweight, each of which is 8 stone, which is defined as 14 pounds.