Barrel of Oil (bbl)

Definition

A barrel of oil (abbreviated bbl) is the standard unit of measurement for crude oil and petroleum products in international markets. One barrel equals 42 US gallons or 158.987 litres (approximately 159 litres). All major oil price benchmarks — Brent crude, WTI, Dubai/Oman — are quoted in US dollars per barrel.

Production volumes are typically expressed in barrels per day (b/d) or million barrels per day (mb/d). World oil production in 2026 is approximately 102–104 mb/d.

Conversion table

The conversion of tonnes to barrels varies by crude grade: lighter crudes (lower density, higher API gravity) yield more barrels per tonne than heavier crudes. For API gravity context, see our entry on sweet crude and API gravity.

Why is oil measured in barrels? The historical origin

The 42-gallon barrel is a historical accident that became a global standard. When commercial oil production began in Pennsylvania's Oil Creek region in 1859, there were no purpose-built oil containers. Early producers used whatever wooden barrels were available — initially 40-gallon whiskey barrels. These leaked, were inconsistent, and created measurement disputes.

In 1872, the Pennsylvania oil industry standardised on the 42-gallon "tierce" barrel — borrowed from the standard herring barrel used by Atlantic fishermen. The extra 2 gallons over the 40-gallon whiskey barrel allowed for evaporation (ullage) during transport. The Petroleum Producers Association formalised this in 1872, and the New York Mercantile Exchange (NYMEX) adopted it when WTI futures trading began. The unit became internationally entrenched through US market dominance.

Today, no crude oil is physically shipped in wooden barrels — it moves in supertankers and pipelines — but the barrel remains the pricing and measurement unit. For the WTI benchmark that uses this unit, see our entry on WTI crude.

What one barrel of crude produces when refined

Crude oil is a mixture of hydrocarbon chains of varying lengths. Refining separates and upgrades these chains into usable products. A typical US refinery barrel (42 gallons in → ~44.5 gallons out, due to processing gain) produces approximately:

These proportions vary by crude type and refinery configuration. A complex refinery (high Nelson Complexity Index) running light sweet crude maximises petrol and diesel output. A simple refinery running heavy sour crude produces more residual fuel oil. For refinery types and complexity, see our refinery glossary entry.

Barrel as the basis for crude benchmarks and pump prices

Crude oil benchmarks — Brent, WTI, Dubai/Oman — are quoted in USD per barrel. These benchmark prices form the foundation of pump prices globally: a refiner buys crude at the benchmark price, processes it, and sells refined products at prices that must cover the barrel cost plus refining, logistics, taxes, and margin.

As a rough rule of thumb, a $10/bbl change in crude price translates to approximately 6–8 cents per litre at the pump (in a lightly-taxed market like the US). In highly-taxed European markets, the same crude move translates to a smaller percentage pump price change because taxes are a fixed absolute amount that dampens the proportional impact. For this transmission mechanism, see our guide on oil price to pump price lag.

For the Brent crude benchmark specifically, see our Brent crude glossary entry. For WTI — the North American benchmark — see our WTI crude entry.