If converting milliliters to grams of pure water, the relationship is one-to-one, so 120 milliliters of pure water is equal to 120 grams. However, when converting any other substance, density must be taken into account when completing the conversion.

Milliliters describes the volume of a substance, while grams discusses the mass or weight. To complete a conversion of 120 milliliters of a different substance into grams, the density must first be known. Density can be expressed in terms of grams per milliliter. For example, if something has a density of 2 grams per milliliter, 120 milliliters would equal 120 x 2, canceling out the milliliters and leaving the expression in grams for a total weight of 240 grams.