Why does an oil barrel hold 159 liters? Tracing the origin of an iconic unit

Today, the oil barrel is the benchmark unit used on global markets to measure and price crude oil. Yet this volume unit—equal to 42 US gallons, i.e. precisely 158.987 liters—may seem surprising for such a strategic product. Why is this old unit still in use, and where does it come from? This article traces its history to explain the origin of the famous oil barrel.

A unit born from 19th-century industry

The history of the oil barrel starts in the 1860s, at the dawn of the US oil industry in Pennsylvania. At the time, crude oil was extracted manually and hauled by horse or wagon to nearby refineries. A practical, standardized container was needed for storage and transport.

Producers naturally turned to containers already used in neighboring industries, adopting wooden casks common in the whisky, beer and salt trades.

The 42-gallon wooden cask (about 159 liters) was widespread. It was sturdy, small enough to be rolled by one person, and easy to stack. Its relatively modest capacity also helped limit losses in case of leaks.

Why 42 gallons (and not 40 or 50)?

The choice of 42 gallons was no accident:

• Optimal weight: a full crude barrel weighs about 136 kg—manageable by two men.
• Consistent capacity: whisky and wine barrels already held 40–45 gallons.
• Commercial standardization: early barrels often contained 40 gallons, but producers added a 2-gallon allowance (≈5%) to cover transport losses (evaporation, leakage), formalizing the 42-gallon barrel.

In 1866, a group of Pennsylvania oil producers officially adopted 42 gallons as the standard unit. The practice spread across the industry and eventually became the global norm still used today.

The barrel: a unit of volume, not weight

Remember: a barrel is a unit of volume, not weight. A barrel contains 158.987 liters of oil, but the exact mass depends on crude density. A light crude barrel can weigh under 120 kg, while a heavy crude barrel may reach 150 kg.

The barrel today in international trade

Physical barrels have vanished, but the term barrel remains the reference for crude prices. For example:

• Brent or WTI are quoted in USD/barrel.
• One barrel yields roughly 73–75 liters of gasoline and 35–40 liters of diesel.

Conversion to liters and other useful units

A lasting cultural and historical legacy

Despite technological change, the barrel continues to symbolize oil. It appears in charts, the press, IEA forecasts and global statistics.

The barrel remains a technical, media and symbolic unit all at once.

Far from arbitrary, the 159-liter barrel was born of practical needs in the 19th century. Its convenient size and standardization helped structure the early oil trade. Today, it still embodies the tradition—and efficiency—of an industry more than 150 years old.