according to page c of this nist document by custom 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 gallons is by custom not by law and b the definition of gallons is for statistical purposes and does not mention for use in trade or commerce this nist document page also states that a barrel of oil is gallons approx 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 barrels gallons