Why does a barrel of oil contain 42 gallons?

A barrel of oil contains 42 gallons largely due to historical and practical reasons established in the 19th century. When oil was first transported in barrels, the 42-gallon size was chosen because it was a manageable volume for handling and shipping. This standardization helped facilitate trade and commerce in the burgeoning oil industry. Over time, the 42-gallon barrel became the industry standard, and it remains so today.