A unit of measurement most commonly used for crude oil, but occasionally for other commodities such as beer and water. A barrel of crude oil is equal to approximately 42 US gallons or 159 litres. The actual measurement depends on the fluid being measured. A barrel of water or beer, for example, is 31 US gallons or 124 litres. The barrel was once commonly used throughout the world but has been replaced for most applications except crude oil everywhere except the US with metric measurements, most commonly the hectolitre (100 litres).