A milligram, abbreviated as mg, is a metric unit of measure that is equal to 1/1000th (one one thousandth) of a gram, while a microgram, abbreviated mcg, is a metric unit of measure that is equal to 1/1000000th (one one millionth) of a gram. One microgram is equal to 0.0010000 milligrams.
Though they carry different values, micrograms and milligrams have similar characteristics. Both micrograms and milligrams can be used to measure both weight and mass. Micrograms are a common unit of measurement in the medical field in the United States even though the U.S. typically doesn't use the metric system.