A barrel of crude oil generally refers to 42 US gallons, which is about 158.98 liters. The quality of crude oil is related to its density. The density of crude oil varies with its quality and temperature, and generally fluctuates between 0.67~ 1.0 1 g/cm3. According to the average proportion of crude oil in the world, 1 ton of crude oil =7.33 barrels, that is, one barrel of crude oil =0. 136 tons.

There is a theory that 42 gallons is one of the imperial standards for sealed wooden barrels promulgated by King charles ii of England in the late14th century, which was later adopted by the British colony of Pennsylvania. Another way of saying this is that 42 gallons was improved from the 40-gallon whisky barrel used in19th century, in order to increase users' trust in delivery, and was finally recognized as a unified standard unit of measurement and trading by the American Petroleum Producers Association in 1872.