No general equation exists for converting international units to milligrams. An international unit is a measurement typically used for medications, vaccinations and vitamin supplements, but the volume of an international unit changes with the individual substance.
International agreement determines how much an international unit is for each substance, and the measurement depends upon the potency of the substance. One international unit of Vitamin A is equivalent to 0.3 micrograms or 0.0003 milligrams. Alternatively, an international unit of Vitamin E is 0.67 micrograms or 0.00067 milligrams.
If you need to know the milligrams contained in an international unit of a pharmaceutical product, ask your pharmacist to find it for you. Information on nutritional supplements is usually available from the manufacturer.