Crude oil is measured in barrels. When crude oil first came into large-scale commercial use in the United States in the 19th century, it was stored in wooden barrels. One barrel equals 42 US gallons, or 159 litres. In some cases crude oil is also measured in tons. The number of barrels contained in each ton vary depending on the type and specific gravity of each crude, however the average number considered would be around 7.33 barrels per each ton.