See Barrel for other uses.

The barrel is the name of several units of measurement.

• A barrel of crude oil or other petroleum product (abbreviated bbl) is 42 US gallons (158.97 litres), or 35 Imperial gallons. The measurment originated in the early Pennsylvania oil fields. Both the 42 gallon barrels (based on the old English wine measure, the tierce) and the 40 gallon (5 US bushels or 151.40 liters) whiskey barrels were used. The 40 gallon barrel was the most common size early, but by 1866 the oil barrel was standardized at 42 gallons. Oil has not been shipped in barrels for a very long time [1], but it's still used as a unit for measurement and pricing.
• In the UK, a standard beer barrel is 36 UK gallons, about 163.7 litres.
• In the US, a standard barrel for liquids is 31.5 US gallons (119.2 litres), half a hogshead. However, a standard beer barrel is 31 US gallons (117.3 litres), the result of tax law definitions.
• In the US, a standard dry barrel is also equivalent to 105 dry quarts (115.6 litres).