oil barrel
Volumeexact by definition
The oil barrel is exactly 42 US gallons, 158.987294928 litres. It is the unit the entire petroleum industry trades, reports and prices in: production is quoted in barrels per day, reserves in billions of barrels, and every crude benchmark is a dollar figure per barrel.
Watch out: No oil has moved in an actual barrel for over a century; it is a unit of account, shipped in pipelines and tankers. Note also that the US beer barrel is 31 gallons and the UK beer barrel 36 imperial gallons, so "barrel" without a qualifier is genuinely ambiguous outside the oil industry.
| 1 bbl | 158.98729 L |
The 42-gallon figure comes from the Pennsylvania oil fields of the 1860s, where producers adopted the existing 42-gallon tierce used for herring and other goods. Coopers already made them, wagons already carried them, and a 40-gallon barrel filled to 42 gave the buyer a margin against leakage in transit.