The "How Many?" website:
http://www.unc.edu/~rowlett/units/index.html
tells all the gory details of units of measurement.
In the US system, the avoirdupois ounce of weight is 1/16 of a pound, which
is 7000 grains. A grain is originally the weight of a barleycorn. The
avoirdupois ounce is about 28.35 grams.
The US fluid ounce is 1/16 of a pint. There are 2 pints in a quart and 4
quarts in a gallon. The US system uses the traditional British wine
gallon, defined as 231 cubic inches by Parliament in 1707. The US fluid
ounce is about 29.57 milliliters.
The British Imperial fluid ounce is 1/20 of an Imperial pint or 1/160 of an
Imperial gallon. The Imperial gallon was defined by Parliament in 1824 to
be exactly 277.42 cubic ounces, which is the volumn of 10 pounds of water
under certain conditions. The imperial fluid ounce is about 28.41 milliliters.
One milliliter of water under certain conditions weighs one gram.
So a US fluid ounce weighs 1.04 avoirdupois ounces and an Imperial fluid
ounce weighs 1.002 avoirdupois ounces.
Jeff