-> In the US an ounce of mass is exactly 28.349 523 125 g. 
 
-> So if water had a density of 0.998 g/mL (a value found in many textbooks) th 
-> one  imperial fluid ounce of water would have a mass of about one 
ounce. 
  
As far as I know, the U.S. and imperial units of mass are equal, except 
that it's conventional in the U.S. to use a ton of 2000 lbs, whereas 
the imperial ton is 2240 lbs. 
  
The imperial "fluid" units were fiddled with in the 1700's as part of 
an effort to make the system more rational and vaguely decimal. An 
imperial gallon of water weighs 10 lbs. This is compatible with having 
a fluid ounce that weighs one ounce by having 20 ounces in a piunt, 
instead of 16. 
  
                    dow 
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