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 to 200 litres (22 to 44 imp gal; 26 to 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. 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 In most countries such usage is obsolescent, increasingly superseded by SI units.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.

In the international oil market context, however, prices in United States dollars per barrel are commonly used, and the term is variously translated, often to derivations of the Latin/Teutonic root fat (for example vat or Fass).

In other commercial connections, barrel sizes such as beer keg volumes also are standardised in many countries.

Some products have a standard weight or volume that constitutes a barrel:

Fluid barrels vary depending on what is being measured and where. In the UK a beer barrel is 36 imperial gallons (43 US gal; 164 L). In the US most fluid barrels (apart from oil) are 31.5 US gallons (26 imp gal; 119 L) (half a hogshead), but a beer barrel is 31 US gallons (26 imp gal; 117 L). The size of beer kegs in the US is based loosely on fractions of the US beer barrel. When referring to beer barrels or kegs in many countries, the term may be used for the commercial package units independent of actual volume, where common range for professional use is 20-60 L, typically a DIN or Euro keg of 50 L.

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