Math & Measurement
International Unit (IU)
Also called: IU, international units
Definition
A substance-specific unit of biological activity that cannot be converted generically to mg or mcg.
What International Unit (IU) means
International Units are defined around biological activity for a particular substance rather than a universal mass relationship.
Why the distinction matters
Because the mass represented by one IU can differ from one substance to another, BACToBasics does not use a generic IU-to-mg or IU-to-mcg conversion rule.
Common confusion
IU is not the same as the numbered markings on a U-100 insulin syringe. One describes biological activity; the other is a syringe volume scale.
See this concept in context
Open the BACToBasics tools and reference pages where this term is used.
Keep learning
Related glossary terms
Math & Measurement
U-100 Syringe Units
Volume markings on a U-100 insulin syringe where 100 units correspond to 1 mL of liquid.
Math & Measurement
Milligram (mg)
A unit of mass equal to one-thousandth of a gram.
Math & Measurement
Microgram (mcg)
A unit of mass equal to one-millionth of a gram, or one-thousandth of a milligram.
Published September 6, 2026. BACToBasics glossary pages are educational definitions only and do not provide medical, dosing, preparation, treatment, or product-selection advice.