Both milliliters (abbreviated "mL") and milligrams ("mg") are common units in the SI system of measurement, more commonly known as the metric system. The link between these two units is the density of a substance. Density is a term that describes the amount of mass of a substance found in a given volume. Scientists working in the metric system typically use units of grams per milliliter (g/mL) for density. Using density, you can convert milliliters to grams. You can then convert to milligrams based on the metric system conversion factor between these two units.
Enter the value of milliliters into the calculator. This is the volume of the substance, or the amount of space it takes up. For example, if you had a beaker holding 28 mL of a liquid, you would enter 28.
Multiply the value you just entered by the density of the substance, in units of grams per milliliter. The result of this calculation is the mass (also commonly called the weight) of that volume of substance, in units of grams. If the liquid had a density of 1.24 g/mL, the calculation would be 28 x 1.24 = 34.72 g.
Multiply the value of grams found in the previous calculation by 1,000. The result will be the number of milligrams of the substance, since there are 1,000 milligrams in a gram. In the example, there would be 34.72 x 1,000 = 34,720 mg.
TL;DR (Too Long; Didn't Read)
Pure water at room temperature has a density of almost exactly 1 g/mL, so the conversion between milliliters and milligrams is simplified.