Oil prices are quoted by the barrel every day, yet almost no oil actually travels in barrels anymore. It moves through pipelines, tankers, and rail cars. The barrel survives purely as a unit: exactly 42 US gallons, which is about 158.99 liters. Why 42? The answer is a piece of nineteenth-century American history that outlived its own container.
When the Pennsylvania oil boom began in 1859, drillers needed containers immediately and grabbed whatever the region already made: wooden casks built for whiskey, salt, fish, and other goods. Cask sizes varied, which made buying and selling messy — a “barrel” of oil could mean anything. The 40-gallon whiskey-style barrel was common, and the story goes that producers settled on 42 gallons partly to standardize on an existing cask size known as the tierce, and partly to include a margin over 40 gallons so buyers would not feel shortchanged by leaks and evaporation in transit.
From cask to convention
By the early 1870s, the Petroleum Producers Association and later the US Geological Survey and Bureau of Mines embraced the 42-gallon barrel as the standard, and the convention hardened into the unit the industry still uses. The wooden cask itself faded quickly — steel drums, tank cars, and pipelines were safer and cheaper — but contracts, taxes, and statistics had already been written in barrels, and units with paperwork behind them are nearly immortal.
Confusingly, the familiar steel drum of the modern world holds 55 US gallons, not 42. A drum of oil is not a barrel of oil, which trips up more than a few students of the industry.
The numbers that flow from 42
The 42-gallon definition generates the industry’s everyday arithmetic. A barrel is 42 US gallons, roughly 159 liters, or about 35 imperial gallons — a reminder that the US and imperial gallons are different units. Production is quoted in barrels per day, and refiners talk of the “refinery gain” by which a barrel of crude yields slightly more than a barrel of refined products, because the products are less dense.
For mental math, two anchors help: a barrel is a bit less than 160 liters, and a cubic meter of oil is about 6.29 barrels. Global oil consumption runs on the order of 100 million barrels per day, a figure that makes the humble cask-derived unit responsible for describing one of the largest material flows on Earth.
A unit only oil uses
Outside petroleum, “barrel” means other things: US federal law defines a beer barrel as 31 gallons, and dry goods historically used yet other barrel sizes. Only oil’s barrel is 42 gallons, which is why careful writing calls it the oil barrel, abbreviated bbl.
When you see oil at some price per barrel, translate it as per 159 liters — and remember you are looking at the ghost of a Pennsylvania whiskey cask.
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