According to page C17 of this NIST document by custom 42 gallons comprise a barrel of crude oil or petroleum products for statistical purposes and this equivalent is recognized for liquids by four States Based on the above it should be noted that a the definition of 42 gallons is by custom not by law and b the definition of 42 gallons is for statistical purposes and does not mention for use in trade or commerce This NIST document page 10 also states that a barrel of oil is 42 gallons approx 159 liters NYMEX does define the capacity of a barrel in terms of gallons according to this Here we see that the trading unit for Light Sweet Crude Oil is specified as 1 000 barrels 42 000 gallons