Barrel (unit)

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A barrel is a unit of volume used for many liquids, and there are several kinds: dry barrels, fluid barrels (for example Britain’s beer barrel and the American beer barrel), and oil barrels. The actual size of a barrel has varied widely over time and by country. Today, common barrel sizes roughly range from 100 to 200 litres (about 22 to 44 US gallons). In many cases, the word drum is used almost interchangeably with barrel.

Origin and use
The term barrel comes from the medieval French baril. Across Europe the size of a “barrel” has ranged from about 100 litres to about 1,000 litres, and the word has taken on different meanings in different languages. In most countries this usage has faded in favor of SI units, though the oil market still often uses the term barrel (bbl) as a standard measure.

Oil barrels and standard measures
In the oil industry, one standard barrel is exactly 42 US gallons, about 159 litres, defined at a reference temperature of 60 °F (15.6 °C) and 14.696 psi. This standard is not a fixed physical container–the actual volume changes with temperature and pressure. Oil contracts and trading often use the symbol bbl, and production is reported in multiples such as Mbbl (thousand barrels) or MMbbl (million barrels). To avoid confusion, some industry groups prefer spelling out thousands or millions instead of using M or MM.

Other barrel sizes
Outside oil, barrels can differ. In the UK, a beer barrel is 36 imperial gallons (about 164 litres). In the US, many non-oil fluid barrels are about 31.5 US gallons (roughly 119 litres), and beer kegs follow related sizes. Today, many liquids are shipped in 200-litre (55 US gallons) drums.

Conversion and measurement
Converting barrels to cubic metres is not exact because oil volumes depend on density and temperature. A common near-term rule is that 1 cubic metre is about 6.2898 oil barrels, but the exact factor varies with the crude. For trading and regulation, exact conversion factors consider temperature, pressure, and the oil’s density. Some regions report oil in cubic metres or tonnes, while others use barrels plus their local standards.

History and context
Oil production in the 1800s led to the need for a standard barrel. By 1866, producers in Pennsylvania agreed to a common measure, and the 42-gallon US barrel was adopted in 1872 (and in 1882 by U.S. agencies). Today, barrels of oil are a core unit in pricing, reporting, and accounting, while many other liquids use different barrel sizes or the metric system.