Barrel (unit)

Lore & Background

Since medieval times, the term barrel as a unit of measure had various meanings throughout Europe, ranging from about 100 litres to about 1,000 litres. The name derived from the French baril, of unknown origin, still used in French and derivations in many other languages. In most countries, such usage is obsolescent, superseded by SI units. In the international oil market, 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 (e.g., vat or Fass).

In the US, dry barrels are defined by specific dimensions: the US dry barrel is 7,056 cubic inches, with stave length 28+1⁄2 in, head diameter 17+1⁄8 in, distance between heads 26 in, and bulge circumference 64 in. The US cranberry barrel is 5,826 cubic inches. Fluid barrels differ: in the UK a beer barrel is 36 imperial gallons; in the US most fluid barrels (apart from oil) are 31.5 US gallons, but a beer barrel is 31 US gallons. Richard III, King of England from 1483 until 1485, defined the wine puncheon as 84 wine gallons and a wine tierce as 42 wine gallons; the 42-gallon tierce became a standard container for shipping various commodities in English colonies by 1700.

In the oil industry, one barrel (bbl) is exactly 42 US gallons. The American Petroleum Institute defines a standard barrel of oil as the amount occupying exactly 1 barrel at 60 °F and 14.696 psi. Oil companies report production using multiples such as Mbbl (thousand barrels) and MMbbl (million barrels), though the Society of Petroleum Engineers recommends spelling out these terms to avoid confusion. Some European companies use kb (kilobarrels), mb (megabarrels), and gb (gigabarrels). The abbreviation 'bbl' may have originated to indicate plural or to avoid confusion with 'bl' for bale; some sources assert it stood for 'blue barrels' from Standard Oil, but the abbreviation was in use before the 1859 U.S. petroleum industry.

Reader's Guide

The barrel remains a significant unit in the oil industry, where it is standardized as 42 US gallons and used for pricing and production reporting worldwide. Its historical evolution from medieval European measures to a modern commercial standard illustrates the persistence of traditional units in specialized contexts. The article notes that the term 'barrel' is often used interchangeably with 'drum' in many connections, and that beer keg volumes are standardized in many countries. The conversion between a standard oil barrel and a standard cubic metre is complicated by differing reference conditions and the expansion coefficient of crude oil, which varies by density. For light oil (density 850 kg/m³), warming from 59 °F to 60 °F might increase volume by about 0.047%; for heavy oil (density 934 kg/m³), by about 0.039%. International commodity exchanges set conversion factors for benchmark crude oils to facilitate trading. The legacy of the barrel is its continued use in global oil markets, despite the general obsolescence of barrel units in most other contexts, where SI units have superseded them.

Did You Know?

• The US dry barrel is defined as 7,056 cubic inches, with specific dimensions for stave length, head diameter, and bulge circumference.
• The abbreviation 'bbl' was in use before the 1859 birth of the U.S. petroleum industry.
• A standard oil barrel is defined at 60 °F and 14.696 psi, not simply as a fixed volume.