Identify the molecule in question and determine the amount of IUs that need to be converted. For example, vitamin amounts are normally reported in IUs; therefore, assume that 1000 IU of Vitamin D3 needs to be converted to micrograms.
Look up the number of micrograms per IU for the specific molecule in question. The conversion factor for the specific molecule in question can be found online, as well as in many medical reference literature. 1 IU of vitamin D3 is reported to equal 0.025 micrograms.
Multiply the conversion factor by the number of IUs to identify the amount of the molecule. For example:
1000 IUs of Vitamin D3 would be equivalent to 25.0 micrograms.
(0.025 mcg)/(1 IU of Vitamin D3) * (1000 IUs) = 25.0 micrograms
Realize that conversion factors are unique to different substances. For example, the same conversion in the Vitamin D example could not be used for other Vitamins. Each conversion factor would need to be identified in reference material for the respective molecules.