What standard volume is represented by one U.S. petroleum barrel?

One U.S. petroleum barrel represents 42 U.S. gallons.

The petroleum barrel became a standard commercial unit in the United States during the nineteenth century, when crude oil was commonly shipped in wooden barrels. The 42-gallon measure was eventually adopted as the industry standard, even after petroleum transport shifted to pipelines, railcars, tankers, and trucks.

A petroleum barrel is about 159 liters. It is a unit of volume, not a guarantee that a physical metal or wooden barrel is present. Production, reserves, refining capacity, and international oil prices are frequently reported in barrels or barrels per day.

The petroleum barrel should not be confused with the U.S. beer barrel, which has a different volume. Refineries also produce many products from one barrel, including gasoline, diesel, jet fuel, liquefied petroleum gases, and asphalt, so the output mix varies with crude quality and refinery design.