One oil barrel is exactly 158.987294928 litres, or 42 US gallons. The 42 is not a rounding of anything — it is a specific historical choice made in the Pennsylvania oil fields in the 1860s and never revised since.
About the units
Early oil producers shipped crude in whatever barrels they could find, and the resulting disputes over quantity led the Petroleum Producers Association to standardise in 1872 on a 42-gallon barrel. The number came from the English tierce, a wine cask of 42 gallons that was already in common use for shipping and which held about as much as a man could roll. The physical barrel disappeared long ago — oil moves by pipeline and tanker — but the unit remained, and it is now the standard quantity in which crude oil is priced globally. It is not used for refined products in most markets, which are sold in litres or gallons.
The exact factor
The derivation is 42 × 3.785411784 = 158.987294928 litres exactly. In cubic metres that is 0.158987294928, so roughly six and a third barrels make a cubic metre. Two useful reference points: a barrel is about 159 litres, and world oil production of roughly 100 million barrels a day is about 15.9 million cubic metres a day. Note that the beer barrel, the dry barrel and the UK barrel are all different units — the oil barrel's 42 gallons is specific to petroleum.
Where you meet this conversion
Oil pricing, production statistics and reserve estimates, all quoted per barrel internationally including in fully metric countries. Refinery capacity is barrels per day. Energy statistics use barrels of oil equivalent as a common unit for comparing gas, coal and renewables, which is a measure of energy content rather than volume and is defined as about 5.8 million BTU. Anyone reading energy data needs to keep the volume barrel and the energy-equivalent barrel apart.