Math & Measurement
Microgram (mcg)
Also called: mcg, µg, ug
Definition
A unit of mass equal to one-millionth of a gram, or one-thousandth of a milligram.
What Microgram (mcg) means
Micrograms are useful when a mass is small enough that writing it in milligrams would require several decimal places.
Why the distinction matters
One milligram equals 1,000 micrograms, so 0.25 mg and 250 mcg represent the same mass.
Common confusion
mcg and mg are both mass units. Neither one tells you the liquid volume or U-100 syringe marking until concentration is known.
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Related glossary terms
Math & Measurement
Milligram (mg)
A unit of mass equal to one-thousandth of a gram.
Math & Measurement
Milliliter (mL)
A metric unit of liquid volume equal to one-thousandth of a liter.
Math & Measurement
Concentration
The amount of a substance contained in a given volume of liquid, such as mg/mL or mcg/mL.
Published September 6, 2026. BACToBasics glossary pages are educational definitions only and do not provide medical, dosing, preparation, treatment, or product-selection advice.