The number of milligrams in 1 cc varies according to the substance to be measured. The metric system uses milligrams to measure weight and cubic centimeters, also known as milliliters, to measure volume.
The density of water is approximately 1 gram per cubic centimeter. The densities of other elements vary greatly. Temperature also affects densities of the materials.
In the medical field, a doctor may prescribe a medication by milligrams. If the medication is in a liquid solution, the concentration in milligrams per cubic centimeter allows it to be dispensed by volume instead of weight. However, concentrations also vary, making it necessary to know the conversion factor.