There are a thousand milligrams in one gram, and there are a thousand micrograms in one milligram. The conversion factor from microgram to milligram a thousandfold. It can be done by expressing the measurement in micrograms, as a decimal number, and moving the decimal point three places to the right.
The conversion from mcg to mg involves expressing a measurement that was originally in units of micrograms in units of milligrams instead. Both micrograms and milligrams are metric units of weight and both are used for very light objects. A microgram is significantly lighter than a milligram. The microgram and the milligram are both based on the gram, a standard metric unit of weight. One ounce is equivalent 28 grams.