Math & Measurement
Milligram (mg)
Also called: mg
Definition
A unit of mass equal to one-thousandth of a gram.
What Milligram (mg) means
Milligrams are commonly used to describe the labeled mass of a compound in a vial or the amount represented in a calculation.
Why the distinction matters
One milligram equals 1,000 micrograms. Converting between mg and mcg changes the number, not the physical amount represented.
Common confusion
mg measures mass. It does not describe liquid volume, syringe units, or biological activity by itself.
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Related glossary terms
Math & Measurement
Microgram (mcg)
A unit of mass equal to one-millionth of a gram, or one-thousandth of a milligram.
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.