Given the mass of an amount of liquid water in milligrams, you may wish to know the volume of that liquid in fluid ounces. This involves the conversion of a unit of mass to a unit of volume. Performing this conversion requires some simple calculation in the case of water.

Calculate the mass of the liquid in terms of grams. For example, you may be working with 800 milligrams of liquid water at 50 degrees Celsius. This translates into 800/1000 = 0.8 grams of water.

Multiply the mass of the water in grams by the appropriate conversion factor. This factor is equal to 0.035. Thus, 0.8 grams of water times the conversion factor is equal to 0.8 x 0.035 = 0.028 fluid ounces.

Record the result with the proper abbreviation. Technically, in the example, this would be "0.028 fl. oz."