`According to page C17 of this NIST document:  http://ts.nist.gov/WeightsAndMeasures/Publications/upload/h4402_appenc.pdf  ``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):  http://ts.nist.gov/WeightsAndMeasures/Metric/upload/fs376-b.pdf   NYMEX does define the capacity of a barrel in terms of gallons according to this:  http://www.nymex.com/CL_spec.aspx  Here we see that the trading unit (for Light Sweet Crude Oil) is specified as ``1,000 barrels (42,000 gallons)``.  `