Since milliliters measure volume and milligrams measure mass, it is necessary to know the density of the substance to convert between the two. The formula mass equals volume times density calculates the conversion.
For example, the density of water at around room temperature is approximately 1,000 kilograms per cubic meter. If a person wants to use the formula, the units must correspond. Kilograms per cubic meter are equivalent to grams per liter. One milliliter is 0.001 liters. After multiplying 0.001 by 1,000, the answer is 1. Liters multiplied by grams per liter become grams. One milliliter of water at a density of 1,000 kilograms per cubic meter is equal to 1 gram or 1,000 milligrams.