What unit of measurement is one-thousandth of a gram?

One-thousandth of a gram is a milligram. The prefix “milli-” means one-thousandth, so 1 milligram equals 0.001 gram, or 10⁻⁶ kilogram. Its standard symbol is mg.

The milligram belongs to the metric system and the International System of Units (SI). Although the gram is not the SI base unit for mass—the kilogram is—the gram and its prefixes are widely used for everyday and scientific measurements. Milligrams are common in medicine, nutrition labels, chemistry, and laboratory work, where quantities are too small to express conveniently in grams.

A frequent mix-up is between milligrams and micrograms. One milligram contains 1,000 micrograms, because the prefix “micro-” means one-millionth of a gram. Milligrams also measure mass, while millilitres measure volume; the two are not interchangeable unless the substance’s density is known. In medication, confusing mg with μg can create a thousandfold dosing error, which is why the abbreviation mcg is often preferred for micrograms in healthcare.