Barrel (unit)
A barrel is one of several units of volume applied in various contexts; there are dry barrels, fluid barrels (such as the UK beer barrel and US beer barrel), oil barrels and so on. For historical reasons the volumes of some barrel units are roughly double the volumes of others; volumes in common usage range from about 100 litres (22 imp gal; 26 US gal) to 200 litres (44 imp gal; 53 US gal). In many connections the term "drum" is used almost interchangeably with "barrel".
Since medieval times the term barrel as a unit of measure has had various meanings throughout Europe, ranging from about 100 litres to 1000 litres, or more in special cases. The name was derived in medieval times from the French baril, of unknown origin, but still in use, both in French and as derivations in many other languages such as Italian, Polish and Spanish. In most countries such usage is obsolescent, increasingly superseded by SI units. As a result, the meaning of corresponding words and related concepts (vat, cask, keg etc.) in other languages often refers to a physical container rather than a known measure.
