A barrel is a unit of volume normally measuring crude oil and other refined elements in the petroleum industry. One barrel contains 42 gallons or 158.9 litres of crude oil. Barrel (often abbreviated as bbl) became the standard measurement unit as early oil producers in the 1800s started using easily available watertight containers such as wooden barrels for drilling oil wells.

Many petroleum players use other related metrics such as boe (Barrels of Oil Equivalent, which is an approximate unit of energy equal to the energy released by burning one barrel or 158.9 litres of crude oil) and boepd (Barrel of Oil Equivalent Per Day).

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