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Milligrams and micrograms are mass; an international unit is biological activity defined per substance — so IU does not convert to mg in general, only per compound.
Labels in this category carry milligrams, micrograms and international units, and the first two are not the same kind of quantity as the third. Milligrams and micrograms are mass: a direct statement of how much material is present. An international unit is a measure of biological activity, defined separately for each substance by an agreed reference preparation.
Because the unit is defined per substance, there is no universal conversion between IU and milligrams. A conversion exists for a given compound and a given reference preparation, and it does not transfer to anything else. Any page offering a general IU-to-mg formula has made an error that will be wrong by a different amount for every compound it is applied to.
A milligram is a thousand micrograms. The abbreviations differ by one character and the quantities differ by a factor of a thousand, which makes this the most consequential typo in the field. When a figure looks implausible by roughly three orders of magnitude, this is the first thing to check rather than the last.
A label states what the vial is intended to contain, not what a test found. The measured value comes from a content or mass assay and is a separate figure. Those two numbers appearing close together is ordinary; the point is that one is a claim and the other is a measurement, and reading the first as if it were the second removes the reason testing exists.
Where a product is sold as a multi-vial kit, the headline figure may describe the kit while a report describes one vial. Dividing one by the other without checking which is which produces a result wrong by the number of vials — typically a factor of ten, which looks alarming and means nothing. Match the unit on the document to the unit on the label before any arithmetic.
After reconstitution the relevant quantity is concentration — mass or activity per unit volume — which depends on how much diluent was added and is therefore not a property of the product at all. Two people reconstituting identical vials differently hold different concentrations. Nothing on the label can tell you this number; only your own volume can.
A report quoting a mass is answering 'how much is here'. A report quoting a purity percentage is answering 'what proportion of what is here is the target'. Confusing the units is the quickest route to believing a document said something it did not, and it is a far more common error than any dispute about the numbers themselves.
Confirm whether the figure is mass or activity. Confirm whether it describes a vial or a kit. Confirm whether it is a label claim or a measured result. And when comparing two documents, confirm both are quoting the same one of those three things before concluding anything from the difference between them.
The claims on this page about milligrams, micrograms and international units, and why they do not convert can be checked against sources outside this site: