Understanding International Units (IU)
Why IU is a measure of biological activity rather than mass, and why an IU to mg factor must come from the specific product.
Last updated November 11, 2026
What an IU is
An international unit (IU) expresses the biological activity of a substance relative to an agreed reference standard. It is a way of describing how active a preparation is, not how heavy it is.
Why IU is not a mass unit
Mass units like mg are the same for every substance: 1 mg of anything weighs the same. An IU is defined separately for each substance, so one IU of one substance corresponds to a different mass from one IU of another. There is no single IU-to-mg number.
Product-specific factors
Where a relationship between IU and mg is used, it belongs to a specific substance and the standard that applies to it. Some product labels state it directly. Manufacturers may state strengths differently, so a figure seen on one website or product should not be carried to another.
What this site does
- It never converts IU to mg automatically.
- In the calculator, an IU amount returns IU/mL.
- If your label states IU per mg, you can enter it as an optional field and the calculator will show the mg/mL equivalent as a second result.
A careful example
Suppose a label states “2 IU per mg” for an imaginary product. Then 50 IU is 25 mg for that product only. For a different product with a different factor, the same 50 IU is a different mass. The factor, not the arithmetic, is what needs verifying.
Where to verify
Read the product’s official labeling on DailyMed or Drugs@FDA.