A unit describing the biological activity of selected substances, not their universal mass. The conversion to micrograms or milligrams depends on the specific substance and its form. Therefore, one IU conversion factor cannot be used for all vitamins.
Activity instead of one universal mass
The international unit refers to the specific biological activity of a substance. Abbreviations IU and IU may appear next to micrograms or milligrams, but there is no single conversion factor common to all vitamins. What matters is the relationship, and sometimes also its form. Therefore, you should not transfer the bill for vitamin D to vitamin E. When comparing offers, it is safest to use the mass value provided by the manufacturer and make sure that it concerns the same portion and the same substance.
The unit must be written together with the name of the substance. The IU number without context is not full information. When the manufacturer also shows micrograms, it is worth keeping both values in the description and not introducing your own conversion from the material regarding another vitamin.
Meaning and application
IU describes the activity of specific substances and is not the common equivalent of a milligram. When comparing labels, the exact name and form are needed. Without this information, a mechanical conversion according to the example from another ingredient may be incorrect.
In practice
Write down the markings of both products and check whether the manufacturer provides the unit of mass in parallel. Use substance-specific data. If records cannot be fairly compared, ask for specifications rather than adopting a one-size-fits-all formula.
Do not use the ratio of one vitamin to all other vitamins. Do not add the IU of different substances as the sum of the strength of the preparation. If the unit symbol is illegible, find the label; a similar entry in the graphic is not a sufficient basis for the decision.