How many U.S. gallons are in the standard petroleum barrel used by the oil industry?

The standard petroleum barrel used by the oil industry contains 42 U.S. gallons.

The modern petroleum barrel developed from nineteenth-century practices in the United States, when producers and traders used wooden barrels to store and transport crude oil. The 42-gallon size became a widely accepted standard even though actual containers varied before standardization.

A petroleum barrel is a unit of volume, not a claim that every physical oil barrel has exactly that capacity. One barrel equals about 159 litres, although the precise conversion depends on the definition of the gallon being used. Oil companies commonly abbreviate it as bbl.

This unit is often confused with the 55-gallon steel drum used for industrial storage. A 55-gallon drum is physically larger and is not the standard unit used in crude-oil production statistics. Production rates such as barrels per day therefore describe volume, not the number of shipping drums.