The number of micrograms in a milliliter of a substance is equal to the density of the substance expressed in micrograms per milliliter. A microgram measures mass, and a milliliter measures volume; they do not convert without the substance's density.
Because micrograms per milliliter is not a standard measurement for density, some conversion may be required to determine the amount of micrograms in a milliliter of a substance. For example, the density of lead is 11.342 grams per milliliter. There are 1 million micrograms in a gram, so there are 11,342,000 micrograms in 1 milliliter of lead. On the other hand, the density of methyl alcohol is 0.785 grams per milliliter, which means that there are 785,000 micrograms in 1 milliliter of methyl alcohol.