For water, which has a density of 1000 milligrams per milliliter, the mass of 250 milliliters is 250,000 milligrams (250 grams). To find the mass of 250 milliliters of a substance in milligrams, one must multiply 250 by the density of the substance.
The mass of a substance in milligrams is equal to its volume in milliliters multiplied by the density in milligrams per milliliter. Usually, densities are given in units of grams per centimeter cubed, where a centimeter cubed is the same thing as a milliliter. Multiplying a density in grams per centimeter cubed by 1000 gives the density in milligrams per milliliter. This result can then be multiplied by a volume in milliliters to give a mass in milligrams.