One U.S. fluid ounce is approximately 30 milliliters, so to convert 100 milliliters to its equivalent in fluid ounces, one simply needs to divide 100 by 30 to get the U.S. fluid-ounce equivalent of 3.33 per 100 milliliters. A fluid ounce should not be confused with an ounce, which is a unit used for measuring mass. However, one can simply use the word "ounce" when the context is obviously referring to fluid measurements.
It must also be noted that the U.S. standard fluid ounce measurement is slightly different from the imperial ounce. The exact U.S. fluid ounce is equivalent to 29.6 milliliters, rounded off to one decimal, whereas an imperial fl. oz. is 28.4 milliliters. The fluid ounce originally refers to a volume of substance that can occupy a wine gallon measuring 231 square inches.
The difference between the U.S. fl. oz. with the imperial ounce came to be when the British Parliament instituted the imperial standards of measurement in 1824. The imperial standard defined the imperial gallon as equivalent to 10 pounds of water. The imperial gallon is divisible into 4 equal quarts, the quart can be divided into 4 equal gills and the gill can be divided into 5 equal ounces. This resulted to the ounce measuring approximately 28.4 milliliters. The U.S. standard fluid ounce, however, stuck with the fluid ounce measurement prior to 1824, which allows for the difference.