Why International Units Cannot Be Converted to mg

If you search for an IU to mg conversion, you will find a table of figures that looks authoritative and is not. This post explains what an international unit actually measures, why the conversion factor belongs to a substance rather than to a number, and how to use a factor without fooling yourself.

What an international unit measures

An international unit, written IU, is a unit of biological activity. It is defined by measuring how much of a substance produces a specified biological effect and comparing that against a reference standard. It is a way of saying “this much effect”, not “this much mass”.

That distinction is the whole problem. Mass is a physical property you can put on a balance. Activity is a property of a substance as measured by a particular assay, and it depends on what the assay measures.

Why the factor is not universal

For any given substance, a large batch has a stable activity per unit mass, so there is a usable conversion factor. Three things prevent that factor from being universal:

It is specific to the substance. A milligram of one peptide and a milligram of another do not carry the same activity. If they did, activity would just be a second name for mass.
It can vary by batch. Manufacturing and storage history change how much of a peptide is correctly folded, and folded peptide is the active form. Two lots of the same product can differ in activity per milligram.
It can vary by assay. Laboratories that measure activity use different methods. The same material measured by two different assays can produce two different factors.

So a figure quoted as “X IU per mg” is always a figure about a specific substance, and often a specific batch and assay. It is not a property of mass.

What that means for a calculator

A converter that offers a universal IU to mg conversion is either wrong or hardcoded to one substance without saying so. The honest design is to make you supply the factor, because you are the only one who knows which substance and batch you have.

The unit converters page asks for the factor in international units per milligram and divides by it. That is the whole calculation:

mass in mg = activity in IU / (IU per mg)

Worked, with numbers invented to show the arithmetic: an activity reading of 2,500 IU at a factor of 100 IU per mg is 25 mg.

2500 IU / 100 IU per mg = 25 mg

The three mistakes worth avoiding

Borrowing a factor from another substance. This is the one that produces a confident, wrong answer. If your factor came from a different peptide, or from the same peptide’s literature rather than your own lot, the result is not an approximation, it is unrelated.

Treating the factor as a physical constant. It is a measurement with uncertainty, tied to a method. Writing it down as though it were like the conversion from mg to mcg is a category error.

Reading an IU as though it were a mass anywhere else. A syringe does not measure IU. A vial label does not state IU per mg without saying which substance. Mass, volume and concentration are all physical quantities with unambiguous units; IU is not one of them.

What to do instead

If you have an activity figure and a mass, the conversion is real arithmetic and the unit converters will do it given the right factor. If you want the mass per vial, that is arithmetic on what the label says, which is what the reconstitution calculator is for. If you want to compare two substances on equal footing, convert to moles rather than activity, because a mole is the same number of molecules whatever the substance is.

A note on what this site will not do

This site publishes no IU per milligram factors, because it has no way to verify one for your specific material and a plausible-looking wrong factor is worse than an absent one. It will do the arithmetic once you supply the factor, and it says on the page that the result is only as good as the factor you gave it.