Barrel is used as a measure of crude oil and other petroleum products. 1 barrel equals 42 US gallons or 158.9873 litres. Alternatively, oil is often measured in cubic metres or in tonnes. Barrel per day (abbreviated BPD, bbl/d, bpd, bd or b/d) is a measurement used to describe the amount of crude oil (measured in barrels) produced or consumed by an entity in one day.

To know more about it, look at what we have already written on this subject