How many gallons in a barrel of oil?

There are 55 gallons in a drum and 42 gallons (1 U.S. gallon = 3.78541178 liters) in a barrel (i.e., 1 barrel = 158.9872948 liters).

Originally there were 40 gallons to a barrel. However, that was changed in the mid-19th century to give a little extra so consumers wouldn't feel "cheated."

A little over 19 gallons of gasoline can be refined from a barrel of oil. Other products (jet fuel, lubricants, etc.) make up the rest.