How this calculator works
An International Unit measures biological activity for a particular substance, not a universal mass. Vitamin D has a verified relationship of 40 IU per microgram. The custom option requires a factor from a reliable source for the exact preparation and form. This page contains no named-drug conversion presets and makes no intake recommendation. The result is an estimate for information only and is not medical advice.
Formula
IU = micrograms x verified IU per microgram. Micrograms = IU/factor. For vitamin D, factor = 40.
Worked example
For vitamin D, 25 micrograms equals 1000 IU, and 10 micrograms equals 400 IU. A custom factor must not be borrowed from a different substance.
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Frequently asked questions
Can all substances use 40 IU per microgram?
No. That relationship is specific to vitamin D. Each other substance needs its own verified factor.
Is a microgram the same as a milligram?
No. One milligram is 1000 micrograms. This input is explicitly in micrograms.
Does a converted amount tell me how much to take?
No. It translates an amount already specified and does not select a dose or an intake target.