Unlike a physical unit, an International Unit has no single fixed relationship to a microgram — the conversion factor depends on which vitamin is being measured.
How it works
Multiplying the entered micrograms by the vitamin-specific conversion factor gives International Units, using the standard factors printed on US supplement labels.
What this does not include
This does not include every vitamin — only the three most commonly searched (D, A, and E) are built in, since each vitamin’s factor is independently defined.
How to use this calculator
- Select the vitamin and enter the amount in micrograms.
Frequently asked questions
Why isn’t there one universal mcg-to-IU conversion?
Because “International Unit” measures a substance’s biological activity, not a fixed physical quantity — the same microgram amount of different vitamins can have very different biological potency.
Why does vitamin D use 40 IU per microgram?
It’s the standard conversion factor set by international agreement and used consistently on US supplement and nutrition labels.
Is this useful for reading a supplement label?
Yes — many labels list one unit but not the other, and this bridges between whichever format a specific product or research source uses.
