Barrel (unit)

A barrel is one of several units of volume applied in various contexts: there are dry barrels; fluid barrels, such as the British beer barrel and American beer barrel; oil barrels, etc. For historical reasons, the volumes of some barrel units are roughly double the volumes of others; volumes in common use range approximately from 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 about 1,000 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 In most countries, such usage is obsolescent, having been superseded by SI units.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 / Germanic root fat (for example vat or Fass).

In other commercial connections, barrel sizes, such as beer keg volumes, 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.^{[citation needed]}

Richard III, King of England from 1483 until 1485, had defined the wine puncheon as a cask holding 84 wine gallons and a wine tierce as holding 42 wine gallons. Custom had made the 42 gallon watertight tierce a standard container for shipping eel, salmon, herring, molasses, wine, whale oil, and many other commodities in the English colonies by 1700. After the American Revolution in 1776, American merchants continued to use the same size barrels.

In the oil industry, one barrel (unit symbol bbl) is a unit of volume used for measuring oil defined as exactly 42 US gallons, approximately 159 liters,or 35 imperial gallons.