Barrel of oil

A barrel of oil is 42 US gallons or 159 litres.

The term comes from the size of the old wooden whiskey barrels used to transport oil in the mid nineteenth century. This unit of measurement has remained unchanged despite the replacement of wooden barrels by larger steel barrels, and the replacement of the latter by pipelines.

Barrels of oil per day (bpd) is a standard unit of oil production.

As oil and gas are often produced from the same fields, it is common to measure reserves and production in barrels of oil equivalent.