Free arithmetic tool
Mass & IU Converter
Convert between micrograms and milligrams, or use a product-specific factor to include international units. IU has no universal mass conversion.
Enter a value and choose both units to see the conversion.
How these units relate
1 mg = 1000 mcg
Pure mass — the same for every substance.
IU ↔ mass varies
An IU measures activity, not mass— use each substance's own factor.
Micrograms and milligrams are both units of mass, so converting between them is a fixed factor of 1,000 (1 mg = 1,000 mcg). An international unit (IU), by contrast, measures biological activity, not weight — so the same number of IU can map to different milligram amounts depending on the substance. That is why every IU conversion above routes through milligrams using an IU-per-mg factor explicitly supplied for the specific product. No default factor is safe across substances or preparations.
Educational reference only — not a recommendation to use, inject, start, stop, or change any substance or medication. Confirm the correct IU-per-mg factor against your product's labeling and your prescriber's guidance.
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Frequently asked questions
How do I convert mcg to mg?
Divide micrograms by 1,000. There are 1,000 mcg in 1 mg, so 500 mcg equals 0.5 mg and 250 mcg equals 0.25 mg. This is a fixed mass conversion that is the same for every substance.
How do I convert mg to IU?
Multiply milligrams by the product-specific IU-per-mg factor. Unlike mcg↔mg, the IU↔mass relationship is not universal. Use only a factor explicitly supplied for the specific product and confirm it with a pharmacist or prescriber.
Why is there no default IU-per-mg factor?
An IU measures biological activity rather than mass, and the relationship varies by substance and preparation. A default could silently produce the wrong conversion, so this tool requires the factor supplied for the specific product.
Why does IU conversion need a separate factor?
An international unit (IU) measures biological activity, not mass, so the same number of IU can correspond to different milligram amounts depending on the substance. mcg and mg are pure mass units that convert directly, but converting to or from IU requires a substance-specific IU-per-mg factor.
Is 1000 mcg the same as 1 mg?
Yes. 1,000 micrograms (mcg) equals exactly 1 milligram (mg). This conversion never changes and applies to any substance because both are units of mass.
Should I rely on this converter for dosing?
Treat the result as a general educational estimate, not medical advice. Verify the IU-per-mg factor against your product's labeling and confirm any dose with your prescriber or clinician before acting on it.