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.
The International System of Units (SI) is the most widely used system of measurement in the world. It is the modern form of the metric system and is made up of a set of base units and prefixes. Each of the base units represents a particular physical property of an object. The base unit for mass is kilograms (kg). Kilograms are 1,000 times larger than grams, making them 1,000,000,000 times larger than micrograms.
The prefixes are added to show that the unit being used is a multiple of the original, either larger or smaller. No matter what base unit, or physical property, is being measured, the same prefixes are used. Whether talking about mass, volume or length, for example, a multiple of 10 has the prefix deca, while a multiple of a tenth, or 10 times smaller, has the prefix deci. A microgram is one million times smaller than a gram and the opposite multiple of a microgram — or one million times larger than a gram — is a megagram.
The scientific abbreviation used for a microgram is µg, using the symbol for the Greek letter mu. In the United States, it was recommended that mcg be used instead of the scientific abbreviation. This recommendation was based on mistakes being made when the abbreviation was hand written and the µ was being misread as an m. When dealing with doses of medicine, this mistake can lead to a one thousand times overdose being administered to the patient.
All metric measurements are taken on the base 10 system, making conversions between the units quite easy. To convert milligrams to micrograms, you must multiply the mass by 1,000. For example, if you have a mass of 2.5 mg, by multiplying by 1,000, you would have 2,500 ug. To convert in the other direction, you must divide by 1,000. The same process is followed to convert to and from grams, except you would multiply or divide by one million.