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See Barrel (disambiguation) for other uses.

The barrel is the name of several units of volume:

• Oil barrel: 42 US gallons, 158.9873 litres,^[1] or 34.97231575 Imperial (UK) gallons.
• UK beer barrel: 36 UK gallons (163.7 litres).
• US beer barrel: 31 US gallons (117.3 litres), the result of tax law definitions.
• US non-beer liquid barrel: 31½ US gallons (119.2 litres), or half a hogshead.
• US dry barrel: 105 dry quarts (115.6 litres).

Standard Oil Company blue barrel

The standard oil barrel is used in the United States for crude oil or other petroleum products. Elsewhere, oil is more commonly measured in cubic metres (m³) or in tonnes (t), with tonnes more often being used by companies which ship most of their oil by sea.^{[citation needed]}. However, many international companies convert all of their oil production volumes to barrels to consolidate them for global reporting purposes since most multinationals are American in origin and financial analysts have come to expect it in company reports. On the other hand, many European companies convert all of their production to tonnes.

This size of barrel is largely unique to the oil industry, since other sizes of barrel were used by other industries in the United States, and most other countries have converted to the metric system. Commonwealth countries, including Britain and Canada, have almost universally converted to SI since none of the fluid measures in the British imperial system were the same size as the equivalent United States customary units, including the imperial gallon and imperial barrel. However, many smaller and/or poorer countries which do not have the technical expertise to develop their own domestic oil industry standards use the American oil barrel for the sake of convenience.

The measurement originated in the early Pennsylvania oil fields. In the early 1860s, when oil production began, there was no standard container for oil, so oil and petroleum products were stored and transported in barrels of all different shapes and sizes (barrels for beer, fish, molasses, turpentine, etc.). Both the 42-U.S.-gallon barrels (based on the old English wine measure, the tierce at 159 litres) and the 40-U.S.-gallon (151.4-litre) whiskey barrels were used. The 40-gallon whiskey barrel was the most common size used by early oil producers, since they were readily available at the time.

However, the Standard Oil Company shipped its oil in barrels that always contained 42 U.S. gallons, allowing an extra two gallons for evaporation and leakage. As Standard Oil came to monopolize 90% of U.S. oil production, customers began to refuse to accept anything less, and by 1866 the oil barrel was standardized at 42 U.S. gallons.

In 1911, the Standard Oil monopoly was broken up into 34 different companies, but its successor companies continued to grow and came to dominate the world oil trade. Oil has not been shipped in barrels for a long time^[2] but the "blue barrel" is still the standard unit for measurement and pricing of oil in the U.S. today.

The abbreviations 1 Mbbl and 1 MMbbl have historically meant one thousand and one million barrels respectively. They are derived from the Latin "mille" meaning "thousand" rather than the Greek "mega". However, since people are becoming more familiar with computer terminology, this is causing increasing confusion. (In non-industry documentation Mbbl, "megabarrel", can sometimes stand for one million barrels.)

The "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale (see above). Some sources claim that "bbl" originated as a symbol for "blue barrels" delivered by Standard Oil in its early days; this is probably incorrect because there are citations for the symbol at least as early as the late 1700s, long before Standard Oil was founded.^[3]

• 55 gallon drum
• United States customary units
• Imperial unit
• Petroleum pricing
• Barrels per day (BPD)

• Crude Oil and Oil Products Conversions
• Oil Industry Unit Conversions