The amount of milligrams in a milliliter depends on the material being measured. A milliliter is a unit that measures volume, and a milligram is a unit that measures mass.
Both milligrams and milliliters are part of the metric system of measurement. A milliliter is one-thousandth of a liter, and a milligram is one-thousandth of a gram. One gram is the calculated weight of 1 milliliter of water at room temperature; a milliliter can also be described as 1 cubic inch. As of October 2014, the metric system continues to be the world standard for measurements in all industrialized countries except for the United States, where the Imperial system is still in use.