In order to convert milligrams to milliliters, one must know the density of the substance; if the density is measured in milligrams per milliliter, the milligrams of mass of the object being measured are divided by the density of the substance to determine the volume in millimeters. Because density is generally not listed in milligrams per milliliter, it is likely that some calculations must be done to convert the density into the appropriate units.
To convert 1,000 milligrams of milk into millileters, one must first determine the density of the milk, which is 1.03 grams per cubic centimeter. A simple mathematical conversion indicates that this density is equivalent to 1,030 milligrams per millileter because there are 1,000 milligrams in a gram and a cubic centimeter is equal to a millimeter. Dividing the 1,000 milligrams of mass by the density of 1,030 milligrams per millileter results in a volume of 0.97 millilters.