Barrel (oil, bbl)
The oil barrel, symbol bbl, is a unit of volume commonly used in the petroleum industry. Its origins date back to the mid-19th century, when the American oil industry adopted the wooden barrel as a standard for storage and transportation. In 1866, the barrel was officially set at 42 US gallons (about 159 liters) to unify trade and pricing. Although physical barrels are no longer used, the “barrel” remains a widely recognized standard for oil production, consumption, and trading. The oil barrel is commonly referenced in discussions of energy, economics, and industrial development as a non-metric unit of volume.
Milliliter (mL)
The milliliter, written as mL, is a metric unit for volume. It is one-thousandth of a liter or one cubic centimeter. It was established as part of the metric system and is used worldwide in medicine, cooking, and science. Because it measures small amounts, it is ideal for both everyday and laboratory use. It is the same size as a cubic centimeter, so people in different fields can easily understand it. This makes the milliliter one of the most useful and common units of measurement in the liter.