Barrels per day is a common flow rate measurement in the oil and gas industry. One “barrel” is equivalent to 42 US gallons (159 liters) – and became the standard US measurement in 1872. This unit of flow rate provides a practical way of calculating the larger volumes of oil over a longer timeframe.

The 55-gallon steel drums commonly associated with oil storage are not tied to the flow rate calculation of “barrels per day.”