A microgram is a metric unit of mass that is equal to one thousandth of a milligram (mg). In other words, a microgram is one thousand times smaller than a milligram. Micrograms are a million times smaller than grams (g); one gram is equal to one million micrograms. A microgram is one of the smallest units of mass that is used. Many medications are prescribed using micrograms as their unit of measurement.