Oil produced from an oil well is usually measured in number of barrels produced per day. One barrel being equivalent to approximately 159 litres or 42 US gallons. This term is commonly used in the oil spot markets as price of oil is quoted in terms of dollars per barrel.
Example: Iraq produced 3.861 million B/D of crude oil in March 2015.
This also helps in calculating how big an oil company is in terms of business. Even import and export of oil is calculated using the above mentioned standard measure.