Question: how many litres in one barrel?
Answer: When used to denote a volume, one barrel is exactly 42 US gallons and is easily converted to any other unit of volume. As the US gallon since 1893 is defined as 3.785411784 litre, a volume of one barrel is exactly 158.987294928 litres. Using the approximate value 159 litre is about 0.013% off.